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Subsections

Research

Solar Physics

INAF Researchers: A. Bonanno, M. P. Di Mauro, A. F. Lanza,
  D. Spadaro, M. Ternullo, R. Ventura
Catania University Researchers: G. Belvedere, A. C. Lanzafame, L. Paternò,
  F. Zuccarello
PhD Students: L. Contarino, A. Murabito, P. Romano., S. Di Giorgio
 
 
Solar physics studies carried out in Catania cover nearly all the regions of the Sun, from its interior, to the surface and corona, and out to the solar wind. In addition to the traditional activities concerning the systematic patrol of solar activity, the structure and the dynamics of the solar interior, the theoretical investigation about the generation and evolution of magnetic fields, the study of the rotational characteristics have been carried out. Recently, there has been a significant involvement in space missions devoted to the observations of the solar atmosphere, in particular the ESA/NASA Solar and Heliospheric Observatory (SOHO) and the NASA Transition Region and Coronal Explorer (TRACE). This allowed the development of activities concerning the spectroscopic diagnostics and modelling of coronal magnetic structures and of the solar wind source regions. Some solar scientists from Catania are now actively involved in a scientific project selected for the Solar Dynamics Observatory, a cornerstone mission within the NASA 'Living With a Star (LWS)' programme, whose purpose is to understand in more detail the nature and the origin of the solar phenomena and variations that affect life and society.

More specifically, research activities have been conducted according to the following scheme.

Systematic patrol of photospheric and chromospheric activity

The systematic observations of the Sun in white light and H$\alpha $ line have been carried on in the framework of an international collaboration aimed at performing the patrol of solar activity. The data acquired within this project refer to: sunspots, faculae, quiescent and active prominences on the disk and on limb, flares. These data are daily sent to the various international collecting centers and put on the web page of the Observatory [126,141].

A "Flare Warning" campaign recently started, within the international research programme on Space Weather. The campaign is based on the following operations:

The final aim is to recognize in advance and to observe by both ground-based and space telescopes solar events which are capable of producing troubles in satellite payloads, or even in Earth-based instruments [65,88,89,124,126,141]. Moreover, Catania Observatory collaborates to the Global High Resolution H$\alpha $ Network and supports the activities of the French-Italian solar telescope Themis (see sect. 4.10).

Solar wind and coronal mass ejections

D. Spadaro has collaborated to the study of the solar wind interaction with the interstellar medium at the boundary of the heliosphere with observations made with UVCS/SOHO, which were used to study the interplanetary He focusing cone within 1 AU. Taken over 2 yr and from differing orbit positions, the series of observations include measurements of He I 584 Å  and Ly${\beta}$ intensities. This is the first time that interstellar helium is observed so near the Sun. Measured intensities were compared to a detailed temperature and density model of interstellar helium in the solar system. The model includes EUV ionization but does not include ionization by electron impact from solar wind electrons. Important day-to-day variations of the intensity were observed, as well as a general decrease as the solar activity rose (both absolute and divided by a model with a constant ionization). This shows that the helium intensity pattern is a very sensitive indicator of the electron density and temperature near the Sun [21].

R. Ventura and D. Spadaro have carried out the analysis of the data acquired with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the SOHO satellite and relevant to the two coronal mass ejections (CME) observed on 2 and 3 November 2000, during the simultaneous observational campaign with various SOHO instruments (MEDOC campaign # 6) held at the Institut d'Astrophysique Spatiale of Orsay-Paris. The two CMEs were related to the eruptions of a large filament structure in an active region close to the West limb of the Sun, and of a prominence near the South Pole, respectively. Intensity and profile of the O VI resonance doublet lines at 1032 and 1037 Å  and of Ly$\beta $ (1026 Å) line, together with the intensity of some other less abundant ions, were observed using the O VI channel of UVCS.

The analysis of these spectroscopic observations has allowed to get information about the distributions of ionic densities and flow velocities in the solar coronal plasma ejected during these transient events. Emission due to ions like C II to O VI indicates a temperature range between $10^{4.5}$ and $10^{5.5}$ K. The simultaneous observations of the two phenomena with EIT/SOHO and LASCO/SOHO helped to clarify the morphology of the bright emission regions. The distribution of the plasma ejected in the interplanetary medium is extremely complex: several strands of plasma irregularly distributed inside the CME structures, whose temporal evolution is significantly different from each other, developed during both events. The velocities determined for each bright element also give a complex picture of the plasma kinematics characterizing these coronal mass ejections. The results obtained provide some guidelines for the development of magnetohydrodynamic models which describe the CME [27].

Figure 2.1: Sequence of EIT He II 304 Å  images of the Sun taken the day before (November 1, 2000 -left panel) the occurrence of the CME, the day of the event (November 2, 2000 - middle panel) and the day after (November 3, 2000 - right panel). The UVCS slit position on November 2 EIT observation (19:19 UT) is also reported together with the OVI and Ly$\beta $ long-slit profiles. The vertical and horizontal axes are the spatial coordinates along the slit and the wavelengths, respectively. Shorter wavelengths are at right and smaller polar angles at the bottom of each panel. The vertical extent is 40 arcmin; the covered wavelength range is 16 Å. The slit center, defined as the slit region of minimum heliocentric distance is located at a P.A. equal to 270$^\circ$. Three bright spots identified and numbered progressively on the basis of their distance from it (in arcmin). The grey scale of the He II 304 Å  has been saturated in order to better distinguish the structures involved in the eruption.
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Spectroscopic diagnostic and modelling of coronal structures

D. Spadaro has discussed the importance of observations and analysis of EUV emission lines for determining the physical structure and dynamics of the solar transition region and corona [84]:

D. Spadaro, A.F Lanza and A.C. Lanzafame, in cooperation with colleagues of the Naval Research Laboratory of Washington, DC and of the NASA Goddard Space Flight Center of Greenbelt, MD, have investigated the hydrodynamic behavior of coronal loops undergoing transient heating. This study was stimulated by a wealth of observational evidence for flows and intensity variations in non-flaring coronal loops, that leads to the conclusion that coronal heating is intrinsically unsteady and concentrated near the chromosphere. They carried out 1-D numerical simulations, in which the timescale assumed for the heating variations (3000 s) is comparable to the coronal radiative cooling time and the assumed heating location and scale height (10 Mm) are consistent with the values derived from TRACE studies. The model loops represents typical active-region loops: 40 to 80 Mm in length, reaching peak temperatures up to 6 MK. Spadaro and co-workers used ARGOS, a state-of-the-art numerical code with adaptive mesh refinement, in order to resolve adequately the dynamic chromospheric-coronal transition region sections of the loop. The new major results from this work are the following. (1) During much of the cooling phase, the loops exhibit densities significantly larger than those predicted by the well-known loop scaling laws (cf. Fig. 2.2), thus potentially explaining recent TRACE observations of overdense loops. (2) Throughout the transient heating interval, downflows appear in the lower transition region (T $\sim$ 0.1 MK) whose key signature would be persistent, redshifted UV and EUV line emission, as have long been observed. (3) Strongly unequal heating in the two legs of the loop drives siphon flows from the more strongly heated footpoint to the other end, thus explaining the substantial bulk flows in loops recently observed by CDS and SUMER. The results of these studies have implications for the understanding of the physical origins of coronal heating and related dynamic phenomena [49,85,86,111,122,123].

A. Lanzafame, in collaboration with colleagues of the University of Strathclyde (Glasgow, UK), Rutherford Appleton Laboratory (UK) and Goddard Space Flight Center (Greenbelt, Maryland) has investigated spectroscopic diagnostics for the solar transition region and corona. The differential emission measure (DEM) of a solar active region is derived from SERTS-89 rocket data between 170 and 450 Å. The integral inversion to infer the DEM distribution from spectral line intensities is performed by the data adaptive smoothing approach. Their analysis takes into account the density dependence of both ionisation fractions and excitation coefficients according to the collisional-radiative theory as implemented in ADAS, the Atomic Data and Analysis Structure. Their strategy aims at checking, using observational data, the validity and limitations of the DEM method used for analysing solar EUV spectra. They investigate what information it is possible to extract, within defined limitations, and how the method can assist in a number of cases, e.g. abundance determination, spectral line identification, intensity predictions, and validation of atomic cross-sections. Using the above data and theory, it is shown that a spurious multiple peak in the DEM distribution between $\log (T_{e})=6.1$ and 6.7, where $T_{e}$ is the electron temperature, may derive from an inaccurate treatment of the population densities of the excited levels and ionisation fractions or from using an integral inversion technique with arbitrary smoothing. Therefore, complex DEM structures, like those proposed for solar and stellar coronae by several authors, must be considered with caution. We address also the issue of systematic differences between iso-electronic sequences and show that these cannot be unambiguously detected in the coronal lines observed by SERTS. Our results indicate that a substantial improvement is required in the atomic modelling of the complex element Fe. The elemental abundance ratio Si/Ne is found to be close to its photospheric value. The same result may be true for the Fe/Ne abundance, but this latter result is uncertain because of the problems found with Fe [19].

Figure 2.2: Average loop density as a function of the average temperature during the evolution of one of the loops obtained from the numerical simulations by Spadaro et al. (dotted line, running counterclockwise) and from the scaling law theory (solid line, see Rosner et al. 1978), gauged according to the initial static equilibrium. Different times are indicated by the following symbols: initial condition ($t=0$) - cross; near the maximum heating rate ($t=1505$ s) - asterisk; near the end of the transient heating ($t=3004$ s) - x; beginning of the downflow ($t=4800$ s) - open diamond; significant downflow $(t=8520$ s) - open triangle; immediately before the catastrophic cooling ($t=9534$ s) - open square; restored initial condition ($t=28021$ s) - open circle.
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Highly energetic transient events in the solar atmosphere

TRACE, owing to the wide range of wavelengths examined and to the high angular and temporal resolution, gives the opportunity to carry out a spatial and temporal analysis of active regions during highly transient phenomena such as flares, providing a new input to the understanding of the mechanisms involved in these events. Zuccarello, Contarino and Romano have studied 3 flares that occurred in AR 8421 between 29 and 30 December 1998 by comparing white light, 1600 ${\bf {\rm\AA} }$ , and 171 ${\bf {\rm\AA} }$ images obtained by TRACE, with BBSO $H_{\alpha}$ images, Mitaka magnetograms and Yohkoh hard X-ray data. The flares, characterized by sudden intensity enhancements in EUV loops and by moss brightenings, have been interpreted in the framework of the two canonical flare models: i.e. simple loop and two ribbon flares. The analysis has shown that the first flare may be interpreted as a two-ribbon flare triggered by reconnection between a sheared arcade and a newly emerging flux tube. The analysis of the second flare strongly supports the model of two-ribbon flares characterized by reconnection occurring at higher and higher levels as time proceeds. Finally, the analysis of the third flare has given the opportunity to relate moss brightening with a probable process of chromospheric evaporation [51].

In the framework of the study on filament eruption, Romano, Contarino and Zuccarello have analyzed the eruption of a prominence, characterized by a helical-like structure and by a non-linear rising motion. The prominence was approximated as a cylindrically curved flux tube and the behaviour of several geometrical parameters was estimated during the activation and the eruption phases. It was determined that, at the onset of the activation, the number $N$ of turns of a magnetic field line over the whole length of the prominence was $\sim 5.0$, while the value of the ratio $P/r_{0}$ between the pitch of the magnetic field lines and the prominence width was $\sim 0.45$. These values are in good agreement with those predicted by the kink mode instability. Moreover, it was found a decrease of the total twist of one helical thread from $\Phi \sim 10 \pi$ to $\Phi \sim 2 \pi$ during the prominence eruption, indicating a relaxation of the magnetic field towards a less twisted configuration. It was concluded that the prominence was initially destabilized by the kink mode instability and, not succeeding in finding a new equilibrium configuration, it erupted [48,82,83,121].

Moreover, Contarino, Romano and Zuccarello described a filament destabilization which occurred on May 5, 2001 in NOAA AR 9445, before a flare event. The analysis is based on $H\alpha$ data acquired by THEMIS operating in IPM mode, $H\alpha$ data and magnetograms obtained at the Big Bear Solar Observatory, MDI magnetograms and $171$ ${\rm\AA}$ images taken by TRACE. Observations at $171$ ${\rm\AA}$ show that $\sim 2.5$ hours before the flare peak, the western part of the EUV filament channel seems to split into two parts. The bifurcation of the filament in the $H\alpha$ line is observed to take place $\sim 1.5$ hours before the flare peak, while one thread of the filament erupts $ \sim 10$ minutes before the peak of the flare. The analysis of longitudinal magnetograms shows the presence of a knot of positive flux inside a region of negative polarity, which coincides with the site of filament bifurcation. The event has been interpreted as occurring in two-steps: the first step, characterized by the appearance of a new magnetic feature and the successive reconnection in the lower atmosphere between its field lines and the field lines of the old arcade sustaining the filament, leads to a new filament channel and to the observed filament bifurcation; the second step, characterized by the eruption of part of the filament lying on the old PIL, leads to a second reconnection, occurring higher in the corona [66,98].

Very energetic flares and CME may strongly influence Space Weather and Earth environment. In the last years, there has been growing interest in the possibility of forecasting flare occurrence, in order to reduce damages to spacecrafts, satellite positioning systems and effects on RF communications. In this context, Ternullo, Contarino, Romano and Zuccarello have carried out a statistical analysis on a number of parameters characterizing sunspot-groups, hosting very energetic flares. The data used in the analysis concern sunspot-groups, were partially collected at Catania Observatory during 1998 - 2002, partially deduced by NOAA reports, and partially on M and X flares obtained by GOES 8 in the soft X-ray range. The results indicate that, for a given group, the probability of flaring is related with its complexity, expressed by its large number of spots and pores, its large area, its membership of some of the most evoluted Zürich types and with the asymmetry of the penumbra of its largest spot. These results are fully compatible with the well-known tendency of the magnetic flux toward reconnection in the presence of a complex magnetic topology. Moreover, a large spot group offers a large ``cross section'' to newly buoyant flux, so that destabilization and flaring are more easily triggered [65,88,89,124,126].

Emergence and evolution of solar active regions

The phase of formation of an active region consists in the buoyancy, at photospheric level, of magnetic flux tubes which cause the formation of one or more pores. Next, these pores may increase their area and number until they coalesce and form one or more sunspots with penumbra and give rise to an AR. However, this process does not always proceed with continuous evolution, because sometimes pores may disappear after 1-2 days. Despite several studies carried out on this topic, it is not yet possible to forecast whether the emergence of a group of pores might give rise to an active region, nor to obtain information on the kind of evolution that the resulting AR may have. This question was the main topic of two PICS (Programme International de Cooperation Scientifique) Workshops, the former held in Catania in September 2001, the latter in Rome in October 2002. During these workshops Contarino, Romano, Spadaro, Ternullo and Zuccarello presented the preliminary works on this topic and the results obtained during THEMIS observing Campaign 2001 and 2002 [98,121].

Sunspots rotation

Zappalà and Zuccarello used the "age selection methodology" (ASM) to study the variability of the sunspot groups angular velocity during the activity cycle. The ASM is able to separate the contribution of Young Sunspot Groups (YSG) from that of Recurrent ones (RSG) in the $\Omega (\theta)$ determination and therefore to evaluate whether the increase in angular velocity during minima (reported in literature using all sunspot groups as tracers), is due to a greater statistical weight of YSG on RSG or whether it reflects a global characteristic of the Sun. The results obtained from the analysis of sunspot groups data collected during the period 1874-1981 (Greenwich Photoheliographic Results) indicate that during minima, besides the fact that the percentage of RSG drops to $\le 5 \%$, both YSG and RSG show the same increase in angular velocity, i.e. 0.16 degrees/day. Comparing these results with those reported in literature and taking into account the internal angular velocity as deduced by p-mode oscillations, it is possible to conclude that the observed higher angular velocity of the Sun during minima concerns several layers of the Sun [92,127].

Internal structure and dynamics

M. P. Di Mauro and co-investigators have concentrated on the use of the method of helioseismic inversions in the attempt to restore the properties of the solar structure in the near-surface region and to provide detailed tests for the equation of state and constrain the envelope helium abundance.

In fact, the detailed structure of the convection zone and of the near-surface region is still quite uncertain, since there remains substantial ambiguity associated with modelling the convective flux, explaining the excitation and damping of the solar oscillations, defining an appropriate equation of state to describe the thermodynamic properties of the solar structure, as well as in the treatment of non-adiabatic effects on the oscillations. In most cases, in fact, the theoretical frequencies are calculated in the adiabatic approximation, which is certainly inadequate in the near-surface region, where the thermal time scale becomes comparable with the oscillation period.

Helioseismic inversions of new precise observations of modes with high harmonic degree ($l < 1000$) obtained from the MDI instrument on the SOHO satellite together with the use of a new suitable procedure to be adopted in the helioseismic inversions of high-degree modes have allowed Di Mauro to obtain precise and well-resolved inferences in the sub-surface layers through the HeII ionization zone and also part of the HeI ionization zone. In order to suppress the uncertainties in the treatment of the surface layers in helioseismic inversion procedures, they introduced the use of a new surface term, developed on the basis of higher-order asymptotic theory of acoustic modes and suitable for the handling of high-degree mode frequencies [14].

Their results have shown that the structure of the near-surface region of the solar model is still a serious concern. In fact, below the photosphere, high-degree modes reveal that there is still a large discrepancy between the model and the observed Sun.

Figure 2.3: The intrinsic difference in the adiabatic exponent $\Gamma _1$ between the Sun and the OPAL (Rogers et al. 1996) equation of state (filled circle) and the Sun and the MHD (Mihalas et al. 1988) equation of state (open triangles), in the sense (Sun) - (model), obtained by inversion of a set of data by Rhodes et al. (1998), which includes high-degree modes, and by considering the new surface term in order to suppress the uncertainties in the treatment of the surface layers in helioseismic inversion procedures, . The vertical errors are $2 \sigma$ propagated errors, while the horizontal bars give a measure of the localization of the averaging kernels.
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Moreover, as it is shown in Fig. 2.3, they found evidence that the OPAL equation of state is able to describe better the plasma conditions in the interior of the Sun below $0.97 R_{\odot}$, whereas the MHD equation of state gives a more accurate description than does the OPAL equation of state in the layers within $0.97 R_{\odot}\leq r\leq 0.99 R_{\odot}$, while above $0.99 R_{\odot}$ the differences become very small and the two equations of state can be considered comparable.

Di Mauro used the same method to provide an estimation of the solar helium abundance in the envelope. The determination of the solar helium abundance by means of helioseimic inversions is directly dependent on the equation of state employed in the reference model. By using the MHD equation of state, Di Mauro et al. (2002) obtained a value of the helioseismic helium abundance in the convection zone $Y_{\rm e}=0.2457\pm0.0005  ,$ consistent within errors with the earlier results. By considering the OPAL equation of state they obtain a value of $Y_{\rm e}=0.2539\pm0.0005 ,$ in quite good agreement or just slightly higher than the earlier findings [14,67].

A. Bonanno, in collaboration with H. Schlattl, and L. Paterno, showed that the inclusion of special relativistic corrections in the revised OPAL and MHD equations of state has a significant impact on the helioseismic determination of the solar age, leading to a remarkably good agreement with the meteoritic value for the solar age [7].
 

Dynamo theory of solar activity

In collaboration, with G. Rüdiger and D. Elstner, AIP, Potsdam, and G. Belvedere, A. Bonanno has developed a high-precision code which solves the kinematic dynamo problem both for given rotation law and meridional flow in the case of a low eddy diffusivity. It has been shown that with a positive $\alpha $-effect concentrated at the bottom of the convection zone, the role of the meridional circulation is crucial in determining the parity of the solution, the form of the butterfly diagram and the phase relationship of the resulting field components [8].
 

The 3$^{rd}$ Italian Solar Research Meeting

The meeting, held in the beautiful island of Vulcano of the Eolian archipelago from September 30th to October 4th 2002 (see Fig. 2.4), is the third one on the Italian Solar Research and follows the successful previous two, which were held in Rome in 1998 and L'Aquila in 2000.

Figure 2.4: Poster of the The 3$^{rd}$ Italian Solar Research Meeting
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The number of participants increased steadily from the first meeting to this one, with a large presence of young scientists, demonstrating the vitality of the solar research in Italy validly projected in the international framework, in spite of the dark predictions about its end in use some decades ago among the astronomical community. The renaissance of the solar physics in these last fifteen years, which coincided with a renewed interest of the young scientists in its study, certainly reflects the successes of the space missions devoted to the study of the Sun, the extraordinary discoveries of the helioseismology, and more recently the opening of the very promising field on the Sun-Earth relationships, called space-weather in modern times.

The meeting, mainly sponsored by the Catania Astrophysical Observatory of the National Institute for Astrophysics, the Department of Physics and Astronomy and the Faculty of Sciences of the Catania University, was attended by 73 participants who contributed with 63 articles published in the Memorie della Società Astronomica Italiana, among which there is a limited number of invited review articles to focus the most important problems in the various aspects of the solar research.

The five days of the meeting were devoted for discussing, in an informal and friendly atmosphere, the most recent problems concerning the interior of the Sun, its atmosphere, the magnetic activity, heating of chromosphere and corona, solar wind and interplanetary space, space-weather, climate global changes, instrumentation, archives, and the future solar space missions.

At its opening, the meeting enjoyed a simple ceremony for celebrating two events, the retirement of Giovanni Godoli and the 70th birthday of Giancarlo Noci. Both scientists have significantly contributed to the development of solar physics in Italy and played an important role as teachers and guides for the young solar physicists in Italy. A few words for illustrating the career, scientific work and the human aspects of Giovanni Godoli were said by Marcello Rodonò, Santo Catalano and Lucio Paternò, while Giannina Poletto did the same for Giancarlo Noci. Two memorial silver plates were offered by the LOC to the two honoured scientists.

More information on the meeting is available through the web page at:
http://web.ct.astro.it/eolian-2002/

Stellar physics

Magnetic activity and variability

INAF Researchers: I. Busà, S. Catalano, G. Catanzaro, G. Cutispoto, A. Frasca,
  A. F. Lanza, F. Leone, G. Leto, E. Marilli, S. Messina,
  I. Pagano
University Researchers: G. Belvedere, A. C. Lanzafame, M. Rodonò
IRA Researchers: C. Buemi, P. Leto, C. Trigilio, G. Umana
PhD Students: K. Biazzo
Fellow: G. Marino
 

Structure and modelling of the stellar chromospheres and coronae

Busà has worked on the study of physical and thermodynamic structure of the atmosphere, from the photosphere up to the mechanically heated layers of the chromosphere, for selected target stars representing different levels of stellar activity. In collaboration with a group in Naples, she has worked on the NLTE modelling of radiative transfer, including line-blanketing, and investigated the diagnostic power of spectroscopic lines such as the Ca IRT triplet and the Na I doublet. To this purpose, high-resolution spectroscopic observations of a sample of 40 late-type stars, spanning from basal to the highest level of activity, have been obtained at Telescopio Nazionale Galileo by means of the SARG spectrograph [112,93].

A. Lanzafame, in collaboration with colleagues of the University of Strathclyde, Rutherford Appleton Laboratory and Goddard Space Flight Center, investigated spectroscopic diagnostics for the solar transition region and corona finding also interesting results for the analysis of stellar coronae. In particular, it is shown that a spurious multiple peak in the DEM distribution between $\log (T_{e})=6.1$ and 6.7, where $T_{e}$ is the electron temperature, may derive from an inaccurate treatment of the population densities of the excited levels and ionisation fractions or from using an integral inversion technique with arbitrary smoothing. Therefore, complex DEM structures, like those proposed for solar and stellar coronae by several authors, must be considered with caution [19].

Pagano, in collaboration with J. Linsky of JILA and University of Colorado and other colleagues has obtained and analysed high resolution echelle spectra of $\alpha $ Centauri by means of the STIS spectrograph on board of the Hubble Space Telescope, deriving information on the upper chromosphere of this interesting solar-like star [75].

Magnetic structures in the photospheres, chromospheres and coronae of single stars and close binary systems

Mapping of the atmospheric inhomogeneities of HR 1099. Messina, Lanza and Rodonò have collaborated to a multiwavelength study of the RS CVn binary HR 1099 (V711 Tau). Simultaneous and continuous observations of H$\alpha $, H$\beta $, HeI D$_{3}$, NaI D$_{1}$,D$_{2}$ doublet and the CaII H & K lines have been performed. The spectroscopic observations were obtained during the MUSICOS 1998 campaign involving several observatories and instruments, both echelle and long-slit spectrographs. During this campaign, HR 1099 was observed almost continuously for more than 8 orbits of 2.8 days each. Two large optical flares were observed, both showing an increase in the emission of H$\alpha $, CaII H & K, H$\beta $ and HeI D$_{3}$ and a strong filling-in of the NaI D$_{1}$,D$_{2}$ doublet (Fig. 2.5). Contemporary photometric observations were carried out with the robotic telescopes APT-80 of Catania and Phoenix-25 of Fairborn Observatories. Maps of the distribution of the spotted regions on the photosphere of the binary components were derived by using the Maximum Entropy and Tikhonov regulatization criteria (Fig. 2.6). Rotational modulation was observed in H$\alpha $ and HeI D$_{3}$ in anti-correlation with the photometric light curves. Both flares took place at the same binary phase (0.85), suggesting these events took place in the same active region. Simultaneous X-ray observations, performed by ASM on board RXTE, show several flare-like events, some of which correlate well with the observed optical flares. Rotational modulation in the X-ray light curve has been detected with minimum flux when the less active G5V star is in front. A possible periodicity in the X-ray flare-like events was also found [42].

Figure 2.5: The observed line profiles of H$\alpha $ (left panel), HeI D$_{3}$, and NaI D$_{1}$,D$_{2}$ doublet (middle panel), H$\beta $ (upper right panel) and Ca II h&k (lower right panel) of the first monitored flare on HR 1099 starting at JD 2451145.51, arranged in order of increasing orbital phase from bottom to top.
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Figure 2.6: Top panel: V-band light curve (filled dots) of HR 1099 in 1998.89 fitted by the Maximum Entropy spot model (continuous line). The flux was normalized to the brightest magnitude (V $_{\rm unsp}=5.744$ at phase=0.21). The residuals ($\Delta F / F$) between the observed and synthetized light curves are also plotted vs. phase. Bottom panel: Maps of the distribution of the spot filling factors at five rotation phases (equator-on orthographic projection). Spots located at latitude below $ -33^{\circ}$ cannot contribute to the flux because the inclination of the star's rotation axis is $33^{\circ}$.
\begin{figure}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./messina/sergio8.ps,height=12cm,angle=90}}\end{figure}

 
Measurement of starspot temperatures. It has been recently proven by Gray and collaborators that line-depth ratios (LDR) are a powerful tool for temperature discrimination, capable of resolving differences $\leq 10$K. In the hypothesis that in slowly-rotating stars the passage of dark spots produces modulation of the center line depth of different amount in lines of different sensitivity to temperature, S. Catalano, Biazzo, Frasca and Marilli have made test observations on three active binaries of RS CVn type. Based on observations made at Catania Astrophysical Observatory at a resolution $R=14 000$, they have shown that line-depth ratios can be effectively used to determine spot temperatures of active binary systems. LDRs of ten line pairs, selected in the 6100-6300Å wavelength range, converted to temperature through the calibration relations derived from observations of about 30 main sequence and giant stars, have led to clear rotational modulation of the average surface temperature with amplitudes of 177 K, 119 K, and 127 K for VY Ari, IM Peg and HK Lac, respectively, with an average estimated error of about 10 K. They have shown that:

Figure 2.7: Upper panel: average effective temperature as a function of rotational phase for VY Ari. Lower panel: simultaneous light curve.
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Table 2.1: Parameters obtained from analytical solutions.
Parameter HK Lac IM Peg VY Ari


$T_{\rm sp}/T_{\rm ph}$

0.795 0.845 0.815
$T_{\rm sp} (K)$ 3788 3943 4007
$\Delta T$ (K) 977 723 909
$A_{\rm rel}$ 0.38 0.36 0.45


     

Figure 2.8: Grids of solutions for the temperature (squares) and light curve (dots) amplitudes for VY Ari.
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Spatial association of spots and plages. The study of spatial association of spots and plages in active binary systems has been continued by S. Catalano, Frasca and Marilli by means of contemporaneous spectroscopic H$\alpha $ and photometric observations. Spectroscopic observations were carried out with the REOSC spectrograph at this Observatory in the spectral range 5860-6700 Å, while photometry have been obtained in various collaborations.

Combining photometric observations obtained at the Ege University Observatory (Tr) and H$\alpha $, performed in summer 2000 on the system RT Lac, rotational modulation of the H$\alpha $ emission has been detected in both components [15]. However, while the H$\alpha $ emission of the more active G5 IV star is brighter over the hemisphere where starspots are mainly located suggesting a close spatial association of spots and plages in this star, the reverse is found for the cooler less massive G9 IV star.

Within the same collaboration, the binary system with solar-type components ER Vul has been studied [36]. The net H$\alpha $ equivalent width, arising from both components, shows a phase-dependent variation. By deconvolving the H$\alpha $ emission contributions, they show that the more active component at the epoch of the observation is the secondary one, and that this latter is also responsible for the rotational modulation of H$\alpha $ emission, interpreted as due to chromospheric plages.

Results on the systematic annual H$\alpha $ and photometric observations obtained at Catania Astrophysical Observatory from 1989 to 1997 for HK Lac have been reported. H$\alpha $ excess-emission, evaluated with the spectral synthesis method, appear in all cases anti-correlated with the $V$ band light curves. From model solutions of light curve obtained in the hypothesis of circular spots the usual parameters $T_{s}/T_{*}$, $R_{S}$, $\varphi$ and $\lambda$ for the dark spots have been derived, where $\varphi$ and $\lambda$ are the latitude and the longitude of the center of the spot of radius $R_{S}$, respectively. For modelling the H$\alpha $ emission equivalent width ($EW_{H\alpha}$) curve, bright spots were considered, assuming the emission flux ratio ( $F_{plage}/F_{chrom}$) between plage and quiet chromosphere as a free parameter [106].

Longitude differences ( $\lambda_{spot}-\lambda_{plage}$) between spot and plages do show a systematic change from the most positive value of +40$^{\circ}$ in 1989 to the most negative value of $-80^{\circ}$ in 1997 (see Fig. 2.9). The authors remark that the change does occur in correspondence of the maximum spot coverage between 1990 and 1992, as displayed by Oláh et al. (1997).

Although not definitely proven, it has been suggested that the change in longitude difference may be associated with the change of magnetic field structure with the activity cycle.

Figure 2.9: Longitude shifts between spots and plages for HK Lac. The two vertical dashed bars mark the time interval during which the minimum average brightness has been observed.
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Systematic observations and activity cycles

Messina in collaboration with Rodonò and Cutispoto [39,118] has continued the systematic search for starspots cycles initiated in late 1992 at the M.G. Fracastoro mountain station with the 80-cm robotic telescope (APT80/1, see sect. 4.9).

The APT80/1 has been entirely devoted to the monitoring of a selected sample of almost fifty known or suspected chromospherically active stars (see Table 2.2). The photometry collected by the APT80/1, supplemented with previous archive data of the Observatory and other observatories, has allowed to obtain the most extended observation database (up to more then 30 years) for several stars, e.g. BY Dra (Fig. 2.10) and V711 Tau (Fig. 2.11), which will enable to address questions concerning activity cycles, active longitudes and surface differential rotation.


Table 2.2: APT80/1 target stars
     
HD 8357   AR Psc   HD 114519   RS CVn   HD 209813   HK Lac  
HD 8358   BI Cet   HD 116204   BM CVn   HD 210334   AR Lac  
HD 9902   BG Psc   HD 116544   IN Vir   HD 216489   IM Peg  
HD 12545   XX Tri   HD 117555   FK Com   HD 218738   KZ And  
HD 17433   VY Ari   HD 119213   CQ UMa   HD 219113   SZ Psc  
HD 20629   XX Ari   HD 134319   IU Dra   HD 224085   II Peg  
HD 21242   UX Ari   HD 136901   UV CrB   HD 234601    
HD 21845   V577 Per   HD 143271     HD 234677   BY Dra  
HD 22403  V837 Tau   HD 143313   MS Ser   HD 250810   CQ Aur  
HD 22468  V711 Tau   HD 144110   EV Dra   HD 283750  V833 Tau  
HD 26337   EI Eri   HD 150708   WW Dra   HD 337518   V511 Lyr  
HD 32008   63 Eri   HD 160538   DR Dra   HD 341475   MM Her  
HD 37394     HD 166181  V815 Her   SAO 91772   LN Peg  
HD 37824  V1149 Ori   HD 167605     SAO 130113   BY Cet  
HD 52452     HD 170527     BD+16 516  V471 Tau  
HD 65626   AE Lyn   HD 171488  V889 Her   BD+16 4908    
HD 86590   DH Leo   HD 175742  V775 Her   BD+20 2465   AD Leo  
HD 106225   HU Vir   HD 179094  V1762 Cyg   BD+43 4305   EV Lac  
HD 106677   DK Dra   HD 184398  V1817 Cyg   BD+48 3686  V383 Lac  
HD 107146     HD 199178  V1794 Cyg   BD+49 2392    
HD 108102   IL Com   HD 200391   ER Vul   BD+61 1211   DM UMa  
HD 112313   IN Com          
     
           


Figure 2.10: V-band photometry of BY Dra. Vertical bars are data from the literature, while dots are our robotic observations. The amplitude of the vertical bars, as well as the scatter in the dots at any epoch represent the peak-to-peak light curve amplitude, which is due to the presence of photospheric spots whose visibility is modulated by the stellar rotation.
\begin{figure}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./messina/sergio5.ps,width=10cm,angle=90}}\end{figure}

Figure 2.11: The same as in Fig. 2.10 but for V711 Tau.
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Lanza, S. Catalano and Rodonò, in collaboration with Ibanoglu, Evren, Tas, Cakirli and Devlen (Ege University Observatory, Bornova, Izmir), analysed a sequence of V-band light curves of the active close binary RT Lacertae (G5+G9 IV), extending from 1965 to 2000, to derive the spot distribution and evolution on the component stars. In the modelling approach, as implemented in a computer code written by Lanza and Rodonò, the Roche geometry and Kurucz's atmospheric models were adopted. The resulting maps of the spot surface distribution were regularized by means of the Maximum Entropy and Tikhonov criteria to take full advantage of the increased geometrical resolution during eclipses. By comparing the maps obtained with these two criteria, it was possible to discriminate between surface features actually required by the data and artifacts introduced by the regularization process. Satisfactory fits were obtained assuming spots on both components and the unspotted V-band luminosity ratio: $L_{G5}/L_{G9  IV} = 0.65 \pm 0.05$. The more massive G5 primary appears to be the most active star in the system and its spotted areas are mainly responsible for the light curve distortions. The yearly spot distributions on both components indicate that their spot patterns consist of two components, one uniformly and the other non-uniformly distributed in longitude, the latter suggesting the presence of preferential longitudes. In particular, spots are concentrated around the substellar points and their antipodes on both stars. The eclipse scanning reveals spots with diameters of $\sim 40^{\circ}$, or possibly smaller, on the hemisphere of the primary star being occulted. The primary shows clear evidence for a short-term activity cycle with a period of  8.5 yr and a possible long-term cycle with a period of approximately 35 yr. The variation of the spot migration rate may be related with surface differential rotation, with a lower limit of $\Delta \Omega /\Omega \sim 3.2\times 10^{-3}$. The G9 IV secondary does not show evidence for an activity cycle, its spot coverage appearing rather constant at  15-20% of its surface. The relative amplitude of its surface differential rotation, as indicated by the variation of the spot migration rate, is $\Delta \Omega /\Omega \sim 2.7 \times
10^{-3}$ [18].

Orbital period modulation and magnetic activity cycle in close binaries

The recent analysis of the long-term activity of RT Lac by Lanza, S. Catalano, Rodonò and collaborators shows that the variation of the orbital period is correlated with the activity level of the primary component (cf. Fig. 2.12. Specifically, the decreases of the orbital period appear to be associated with minimum spottedness and sizeable changes of the surface spot distribution that may be related to increases of the rotation rate of the spot pattern. Conversely, an episode of increase of the orbital period was related to an increase of the spotted area on the primary star. Such results support the recently proposed models that connect the perturbations of the orbital dynamics with the variation of the figure of equilibrium of the active components, due to the operation of non-linear hydromagnetic dynamos in their extended convective envelopes [18] .

Figure 2.12: a) The total spot area $A$ (average between ME and T models) of the primary star in RT Lac, b) the longitude of the centroid of its spot pattern $L_{c}$ and c) the $O-C$ diagram vs. time. The four intervals of approximately constant orbital period described in the text are indicated in panel c) and the times of orbital period changes are marked by the dotted vertical lines. The long-dashed lines in panel c) are linear best fits to the $O-C$ points during intervals of approximately constant orbital period.
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The principal characteristics of the observed orbital period variation in magnetically active close binaries were briefly reviewed and the theoretical models proposed to interpret them were presented by Lanza and Rodonò. In particular, they focussed on the models proposed by Applegate (1992) and Lanza, Rodonò & Rosner (1998) to explain the short-term modulation of the orbital period, as a consequence of the changes of the gravitational quadrupole moment of the active component driven by a cyclic hydromagnetic dynamo. Recent observational results supporting this interpretation and the constraints on the intensity of the internal magnetic fields that are required by the proposed mechanism were discussed. A novel, stringent test of Applegate's model based on future asteroseismic space observations of oscillation mode splitting, was also briefly presented [59,17].

Evolution of stellar magnetic activity and related phenomena

"The Sun in time" project. Messina in collaboration with Guinan (Villanova University, PA/USA) continues to carry out a multi-wavelength study of solar analogues with ages $\sim$ 100 million years to 9 billion years as part of The Sun in Time project. The data for this program are obtained with NASA and ESA satellites such as ASCA, ROSAT, XMM, Chandra, EUVE, FUSE, IUE and HST. Also, observations of most of these stars are being made with Villanova's 0.8 meter robotic telescope (FCAPT) in Arizona. The main scientific goals are (a) to study the solar magnetic dynamo (with rotation as the only variable) and (b) to determine the radiative and magnetic properties of the young Sun with the purpose of constructing irradiance tables to be used to study paleo-planetary atmospheres.

Messina and Guinan [20] have analysed the long-term V-band photometry collected since 1990 as part of the project for a subsample of five young single G0-G5V stars with ages between $\simeq$ 130 Myr and 700 Myr: EK Dra, $\pi^1$ UMa, HN Peg, k$^1$ Cet and BE Cet. Also they include in this study the Pleiades-age ($\simeq$ 130 Myr) K0V star DX Leo (HD82443).

Messina and Guinan found the existence of prominent activity cycles which are the first determined from photometric data (Figs. 2.13-2.14) for all the cited stars. The starspot cycles are compared to those activity cycles derived from CaII H&K emission fluxes and differences are discussed. All the cycle periods, except for EK Dra, fit well the empirical relations with global stellar parameters derived from larger stellar samples. The correlations of the starspot amplitude and frequency ( $\omega_{\rm cyc}$) with age and Rossby number ($R_0$) are investigated.

The following results are also inferred from the present study: i) the fastest rotating stars tend to have longer cycles; ii) the range in the observed cycle lengths seems to converge with stellar age from a maximum dispersion around the Pleiades' age towards the solar cycle value at the Sun's age; iii) the overall short- and long-term photometric variability increases with inverse Rossby number with very high correlation degree, indicating that the level of magnetic activity at least in photosphere is still controlled by the stellar rotation even on the longest time scales (see left panel of Fig. 2.15); iv) the increase with inverse Rossby number of the long-term overall photometric variability seems to level off at the highest rotation rate, which may be interpreted as due to a saturation in the level of photospheric magnetic activity around the activity maximum. v) In a $\omega_{\rm cyc}$-$R_0^{-1}$ diagram, in agreement to what found in analogues studies, the stars tend to dispose along three different branches: inactive, active and superactive (see right panel of Fig. 2.15).

Figure 2.13: Time sequence of V-band magnitudes of DX Leo. Continuous line is a sinusoidal fit to the data with a period P $_{\rm cyc}=3.21\pm0.05$ (yr). Open diamonds are the mean magnitudes computed from individual light curves (V$_{\rm max}+$V$_{\rm min}$)/2 and used in the periodogram analysis.
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Figure 2.14: Time sequence of V-band magnitudes of EK Dra. Continuous line is a sinusoidal fit to the data with a period P $_{\rm cyc}=9.2\pm0.4$ (yr) plus a linear long-term trend, which in the case of a cycle would be longer than 30 years. Open diamonds have same the meaning as in Fig.2.13.
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Figure 2.15: Left panel: The amplitude of the overall V-band variability is plotted vs. log(R$_0^{-1})$. Continuous line represents a cubic fit to the data. The amplitude is computed as the difference between the brightest and faintest observed magnitudes. In the case of EK Dra (D) the filled dot denotes cycle amplitude only, while the asterisk denotes cycle amplitude plus longer-term trend. Right panel: Log( $\omega_{\rm cyc}$/$\Omega$) vs. log(R$_0^{-1}$) for the program stars. The cycles of LQ Hya (G), AB Dor (H) and the Sun are also plotted. Different stellar ages are indicated by different symbols, while each target is identified by the corresponding label as reported in Table 1 of [22]. Smaller symbols denote secondary cycles. Lines are the least-squares fits from Saar & Brandenburg (1999).
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...ep02/rep2002/./messina/sergio4.ps,width=7cm }}
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Lithium abundance versus age and magnetic activity level. It is still not clear if the large spread of Li abundances observed in both clusters and field stars reflects a genuine dispersion of abundances or if it is due to phenomena linked to stellar magnetic activity. Cutispoto discussed observational and theoretical results on Li abundance and concluded that, although activity alone does not seem able to account for the spread observed in Li abundances, it is, at least for late type stars, unsafe to assume that the spread is entirely due to genuine Li depletion [57]. He also analyzed recent data on Be abundance measurements in old clusters, that can shed further light on the activity vs. light elements abundances relationships.

In low-mass main sequence stars, internal structure is determined primarily by stellar mass rather than age. In contrast, surface activity as manifested in X-rays, at least for late-type dwarfs, seems to scale directly with rotation and by consequence with age, but is only slightly dependent on mass. Cutispoto and collaborators presented an analysis of high-resolution spectroscopic and high-precision UBV(RI)$_\mathrm{c}$ photometric observations of a sample of 110 nearby late-F and G-type stars selected for their large rotational velocity. They investigated the relationships between Li abundance, X-ray luminosity, and $v \sin i$. They find that, as expected, the stars in the selected sample show statistically higher Li abundance and activity level than field star samples with similar characteristics, but slower rotation. Surprisingly, however, they also find four rapidly-rotating single main-sequence stars with very low Li abundance. For both single and binary stars they find a large spread of Li abundance for stars with rotation lower than about 18 km s$^{-1}$ The well-established correlation between X-ray luminosity and rotation rate is clearly observed. All single unevolved solar-type stars with $v \sin i$ larger than 18 km s$^{-1}$ are strong X-ray emitters and have high Li abundance. Finally, they find also five evolved stars with very low Li abundance that are still rather fast rotators [13,132]. These results confirm the presence of young very active stars close to the Sun, in agreement with recent findings from EUV and X-ray surveys, although the studied sample does not show such extreme characteristics as those selected from EUV and X-ray surveys at the current flux limits [13,40].

Dynamo theory of stellar magnetic activity

Paternò, Belvedere, Kuzanyan (IZMIRAN, Solar-Terrestrial Dept., Moscow) and Lanza investigated the behaviour of a simple thin-shell $\alpha\Omega$ dynamo model in the asymptotic regime, i.e., characterized by dynamo numbers much larger than the critical one, in order to derive scaling relationships connecting the properties of dynamo waves with global stellar parameters. The proposed approach was applied to stellar models of subgiant and giant stars from K0IV to K1III spectral types in the Hertzsprung-Russell diagram, to predict some characteristics of activity cycles in very active stars. They found that the strength of the dynamo action in such stars is higher than in the Sun. Therefore, larger magnetic field energy and larger spot filling factors were expected, in agreement with observations. The periods of stellar cycles were also estimated and compared with observations. The characteristic times of migration of the starspot belts relative to the cycle period, namely the Hale number, together with the ratio of toroidal to poloidal dynamo magnetic fields, were estimated. From their simplified analysis, they could only derive general trends, but could not perform a direct comparison with the observed properties of particular active stars. These general trends indicate that the cycle periods have a large spread for stars with low rotation rates ($ \sim 1-5$ times the solar one), while they tend to be saturated for stars with high rotation rates ($\sim 5-15$ times solar), for which the periods range from 10 to 20 yr. For such stars, Hale numbers ranged from 1.5 to approximately 4 (the Hale number for the Sun is approximately 1.1), denoting the possible existence of cycles with different periodicities present simultaneously. The ratio of toroidal to poloidal dynamo fields tended to become smaller for increasing rotation rates, indicating a transition from the $\alpha\Omega$ to the $\alpha^{2}\Omega$ type of dynamo. Moreover, the magnetic field filling factors tended to become larger for faster rotation rates, though the effect of the convection zone depth should not be neglected. Their results showed a reasonable agreement with available observations of a sample of active stars they considered [22].

Rüdiger, Elstner (AIP, Potsdam), Lanza and Granzer (also from AIP) faced the question concerning whether dynamo-generated magnetic fields are able to produce such quadrupole terms in the gravitational potential which can explain the observed cyclic orbital variation of RS CVn stars. They started with spherical dynamo models with outer convection zones but without any differential rotation, i.e. with $\alpha^{2}$-dynamos which are known as nonoscillating. With the known anisotropic $\alpha $-tensor of rapidly rotating stars the magnetic modes with the lowest dynamo numbers are nonaxisymmetric with a slow azimuthal drift. They also found, however, stable (i.e. with the lowest dynamo number) axisymmetric oscillating modes but only for a very special, highly inhomogeneous $\alpha $-tensor. The dynamo model was a linear one with an arbitrary field amplitude which could be scaled in order to reproduce the observations. The star proved to be as prolate during the maximum of the toroidal field energy and it proved to be as oblate during the maximum of the poloidal field energy. In the time average the influence of the toroidal field dominated and the star is slightly prolate. From the computed temporal variations of the gravitation quadrupole moment a magnetic field of more than $10^{5}$ G is needed in order to produce a period modulation of order $10^{-5}$ which has been observed [24].

G. Belvedere, in collaboration with Zaqarashvili and Javakhishvili (University of St. Andrews, Scotland and the Abastumani Astrophysical Observatory, Tbilisi, Georgia) proposed a new model to explain the enhanced magnetic activity observed in tidally interacting binary systems. It is based on the assumption that a torsional Alfven wave may be excited and amplified inside the active component by means of a parametric resonance mechanism. Specifically, they suppose that the deviation of the active component from spherical symmetry due to the tidal influence of the companion leads to stellar pulsation in its fundamental mode. It is shown that stellar radial pulsation amplifies torsional Alfven waves in a dipole-like magnetic field, buried in the interior, according to the recently proposed swing wave-wave interaction. Then amplified Alfven waves lead to the onset of large-scale torsional oscillations, and magnetic flux tubes arising toward the surface, owing to magnetic buoyancy, diffuse into the atmosphere producing enhanced chromospheric and coronal emission [29].

Space projects to study stellar activity and variability

The researchers in Catania are involved in the design phase and development of several space projects to study the atmospheres of magnetically active stars in UV and optical bands. Moreover ground based monitoring of some of the targets of the asteroseismology program of the space mission COROT is performed using the telescopes at the M.G. Fracastoro mountain station (see 3.1).

Stellar oscillations and asteroseismology

INAF Researchers: A. Bonanno, M. P. Di Mauro, A. Frasca, A. F. Lanza,
  R. Ventura
University Researchers: L. Paternó, M. Rodonò
Student: G. Mignemi
   

In a review paper, Paternò, Di Mauro and Ventura examined the present status of the asteroseismology, developed in recent years along the guidelines of its mother discipline, the helioseismology. After an introductory history about solar and stellar small amplitude oscillations, the authors recall the basic physics of stellar pulsations and how these can be used to infer the structural and dynamical properties of the Sun and stars. In the framework of their location in the HR-diagram, the authors deal with the several classes of small amplitude pulsators useful for asteroseismic studies. The methodologies concerning the pulsation data inversion, namely the methods for deducing the internal structure and dynamics of the stars from their oscillation spectra, are illustrated with application to the Sun and stars [61].

A major activity of the stellar oscillation group during the year has been the investigation of stars suitable for seismological analysis which has been selected as primary science targets for ground-based observational campaigns or for upcoming space missions. In order to assess the information that is and will be available from such observations, Di Mauro in collaboration with L. Paternò (University of Catania), J. Christensen-Dalsgaard and A. Miglio of the University of Aarhus (DK), H. Kjeldsen and F. Pijpers of the TAC-Aarhus (DK), M. J. Thompson of the Imperial College, London (UK), T. Bedding of the University of Sydney and Mario J.P.F.G. Monteiro of the University of Porto (Portugal), has developed a study on the properties of the internal structure of stars which show solar-like pulsations which, similarly to those observed in the Sun, are believed to be excited stochastically by turbulent convection.

One of the activities of Di Mauro in the team is the production of evolutive models by using the evolution code of Christensen-Dalsgaard by adopting updated microphysics and recent measuments of the observed basic parameters. The code can produce models which include also diffusion of heavy elements and overshooting from the convective core during the main-sequence phase. The structure models which match the observed properties of the star considered are then selected for the calculation of the adiabatic oscillation frequencies by using the Aarhus oscillation code.

In particular Di Mauro has considered theoretical investigation of the stars $\eta $ Bootis and of $\beta $ Hydri.

The subgiant HR 5235, better known as $\eta $ Bootis, is a well-studied bright star of spectral type G0 IV, on which observations have recently confirmed the presence of solar-like oscillations with a spectrum characterized by a large frequency separation $\Delta \nu=(40.47 \pm 0.05)   \mu {\mathrm{Hz}}$ and a small frequency separation $\delta \nu=(3.06\pm 0.14)   \mu {\mathrm{Hz}}$.

Figure 2.16: Evolutionary tracks for $\eta $ Bootis plotted in an H-R diagram, calculated for increasing values of the extension of overshooting from the convective stellar core. Models are calculated with $M=1.7 M_{\odot}$ and assuming the observed metallicity ($Z=0.04$). The rectangle defines the one-sigma error box for the observed luminosity and effective temperature of the star.
\begin{figure}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./dimauro/Fig.3.ps,width=10cm}}
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The theoretical calculations identifies $\eta $ Boo as being in the post-main-sequence phase of evolution with a mass in the range $M=1.64-1.75 M_{\odot}$. It has a helium core, having exhausted its central hydrogen, and it is in the shell-hydrogen-burning phase, evolving toward the red giant branch. The models have a convective envelope extending from the base located at about $r_{\rm cb}\simeq0.84R$ into the photosphere. Models which fit the position of $\eta $ Boo in the H-R diagram may have a maximum overshooting extent of $\ell_{\rm ov}=0.25H_{p}$. Some of the resulting evolutionary tracks plotted in an H-R diagram computed for $\eta $ Bootis with and without overshooting from the convective core are plotted in Fig. 2.16.
Figure 2.17: Echelle diagram based on observed and computed frequencies for $\eta $ Boo. The filled symbols show observed frequencies while the open symbols show computed frequencies for a model which neglects overshooting. Circles are used for modes with $l=0$, triangles for $l=1$, squares for $l=2$, diamonds for $l=3$. The size of the open symbols indicates the relative surface amplitude of oscillation of the modes. Crosses are employed for modes with small predicted amplitude (e.g. g-modes).
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The comparison between theoretical and observed oscillation spectra indicates that both models with and without convective-core overshooting can be constructed which are consistent with the observed frequency separations.

The echelle diagram of Fig. 2.17 has been obtained for a model which does not include overshooting. It shows that the frequencies calculated for $l=0$ and $l=2$ are consistent with observations (filled symbols). The model frequencies for modes with $l=1$ are strongly affected by avoided crossings and they deviate from the expected asymptotical behaviour. From the study of Di Mauro et al. [41], it can be concluded that the observations indicate two possible main evolutionary scenarios for $\eta $ Boo: (i) a more evolved star without core overshooting and whose oscillation spectrum contains frequencies of nonradial modes with mixed character due to avoided crossings; (ii) a less evolved star which includes overshooting from the convective core and has p modes in the observed frequency range which show no mixed character and follow the asymptotic theory. Intermediate cases are also possible as represented by a model which includes modest convective overshooting from the core and with typical characteristics of both the two above mentioned scenarios (see [41]).

The other target considered is HR2021, better known as $\beta $ Hydri, a G2 IV subgiant with a mass close to that of the Sun and for which observations have shown the presence of solar-like oscillations. $\beta $ Hydri is one of the best candidates for asteroseismic studies since it is one of the better-observed individual stars other than the Sun, providing accurate estimates of the basic parameters. $\beta $ Hydri is in the post-main-sequence phase of its evolution, with a helium core and a hydrogen-burning shell. The calculations have allowed to establish that its mass is limited to the range $(1.07-1.20) M_{\odot}$, the age is about $(5.2-6.1)$ Gyr and the radius is $R\simeq 2 R_{\odot}$. The theoretical models can reasonably reproduce the observed spectrum. In particular, Di Mauro and collaborators found that the theoretical large separation is about $\Delta\nu\simeq(52-57)   \mu\mathrm{Hz}$, while the theoretical small separation is about $\delta\nu_0\simeq 5   \mu\mathrm{Hz}$, values which are certainly consistent with the observed ones. However, while the evolutionary scenario of $\beta $ Hydri looks already quite well constrained, thanks to the existing spectroscopic and photometric data, it is evident that only more accurate asteroseismic observations will allow further investigation of the properties of the interior of this star [102,119].

Recently, Di Mauro and collaborators started to consider also structure models of rapidly rotating stars, being able to include the effect of fast rotation in stellar modelling and in the calculation of frequencies of global oscillations. In fact, the rotation remove the frequency degeneracy in the azimuthal order $m$ of the oscillation modes and may also significantly modify the equilibrium structure and the evolution of a star.

The effect of rotation in the models has been taken into account by assuming for semplicity an uniform (solid-body) stellar rotation and including the spherically symmetric component of the centrifugal acceleration in the equations for the stellar structure. The effects of the rotation on global oscillations have been calculated by assuming that the angular velocity inside the star is uniform, following a second-order perturbation approach as described by Gough & Thompson (1999). In particular, such study has been applied to two well-observed $\delta$ Scuti stars, namely V480 Tau and $\theta^2$ Tau A, both members of the Hyades open cluster [58,103].

Asteroseismology of hot subdwarf stars

During a photometric campaign dedicated to the hot subdwarf B (sdB) stars, A. Bonanno, Frasca, S. Catalano, Paternó and Mignemi discovered short period oscillations in PG 1613+426 from time-series photometry carried out with the 91-cm Cassegrain telescope. This star, which is brighter than the average of the presently known sdB pulsators, with B = 14.14 mag, has $T_{\rm eff}=34  400$ K and $\log g = 5.97$. Its position is near the hot end of the sdB instability strip, and it is a pulsator with a well-observed peak in the power spectrum at a period of $144.18\pm 0.06$ s. This star seems to be well-suited for high-precision measurements, which could detect a possible multi-mode pulsation behaviour [35].

Orbital period modulation and oscillation in magnetically active close binaries

Lanza and Rodonò re-visited the connection between magnetic activity and orbital period modulation in RS CVn binaries in order to identify possible asteroseismic tests for models that have been proposed to understand their relationship. These models were based on the variation of the gravitational quadrupole moment of the active component, as a consequence of the modification of its internal rotation regime and magnetic field along its activity cycle (Applegate 1992; Lanza et al. 1998). An analysis based on the solar analogy showed that surface magnetic activity might produce shifts, splitting and broadening of the p-mode peaks in the oscillation spectrum of a star. Such effects make the direct detection of the internal structural changes predicted by the models very difficult. However, a time variation of the internal rotation can be detectable because the p-mode rotational splittings are proportional to the azimuthal quantum number $m$, whereas structural and surface effects controlled by the magnetic fields are expected to be $\vert m\vert$ dependent. Therefore, Lanza and Rodonò proposed a method that correlates the variation of the $a_{1}$ splitting coefficients, that sample the internal rotation, with the orbital period variation in order to test the predictions of the models. An illustrative example of the expected changes is shown is Fig. 2.18 for a model binary system consisting of two main-sequence stars of mass 0.7 M$_{\odot}$ on a 12-hr orbit, which represents a typical short-period RS CVn system. Indeed, the RS CVn short-period systems, such as V 471 Tau, RT And or CG Cyg, were found by Lanza and Rodonò to be the most suitable objects for such a test because surface magnetic effects are likely to be less prominent than in classical or long-period RS CVn binaries [17].

Figure 2.18: The variation of the splitting coefficients $\Delta a_{1}$ for three p-modes versus the relative angular velocity change in the inner shell of the convection zone $\Delta \Omega_{i}/\Omega_{0}$. The splitting coefficients refers to high-order modes with $l=1$ (solid line), $l=2$ (dashed line) and $l=3$ (dot-dashed line), respectively. The upper panel reports the results for $r_{s}/R=0.75$, the middle one for $r_{s}/R=0.80$ and the lower one for $r_{s}/R=0.85$, respectively, where $r_{s}$ is the radius at the boundary between the two shells partecipating to the exchange of angular momentum during the cycle of activity and $R$ is the radius of the star. On the right scale, the relative change of the orbital period of the model binary system considered by Lanza and Rodonò is reported.
\begin{figure*}\centerline{
\psfig{file=/home/gle/oacrep02/rep2002/./lanza/lanzaf2.ps,width=8cm}}\end{figure*}

Chemical composition studies and chemically peculiar stars

INAF Researchers: I. Busà, S. Catalano, G. Catanzaro, F. Leone
University Researchers: F. A. Catalano
IRA Researchers: P. Leto

Extremely metal-poor stars

Busà is involved in the 0Z project (http://www.hs.uni-hamburg.de/DE/For/Gal/Met/0Z.html), a large observational program to find extremely metal poor stars with high-dispersion abundance analyses with the aim of shedding light on the early epoch of the formation of the Galaxy, the chemical evolution of the Galaxy, and related issues. This is being accomplished by observing candidate stars from the Hamburg/ESO Survey (HES) (which has a two times higher effective yield of extremely metal poor stars than the existing HK survey) with Keck I/HIRES, and, starting in 2002, also with Magellan 1.

Extremely metal-poor stars show a large star-to-star scatter in absolute abundances as well as abundance ratios for a variety of elements, e.g., the neutron-capture elements (e.g., McWilliam et al. 1995; Ryan et al. 1996). This phenomenon can most easily be explained in terms of local enrichment of the primordial ISM by a small number of exploding massive stars (Audouze & Silk 1995). As the amount of newly synthesized elements often show large variations with mass (or other parameters like rotation and metallicity) of the exploding star, the abundances might have varied extensively throughout the Halo ISM. Subsequent star formation then preserved these chemical inhomogeneities on the surfaces of low-mass stars. Hence, studying these stars by statistical means, especially by displaying them in diagrams relating different abundance ratios (i.e. correlation diagrams), should reveal interesting information on the production sites of the elements.

Simulations of the early, inhomogeneous Galactic halo (Karlsson & Gustafsson 2001) show that the distributions of extremely metal-poor stars in correlation diagrams are insensitive to any other parameter than the supernova yields. Specific variations in the yields produce certain stellar patterns in the diagrams. Thus, we can distinguish theoretical yield calculations merely by studying the patterns. Furthermore, by quantitatively comparing an observed pattern with simulations it should be possible to reconstruct supernova yield ratios, or even absolute yields assuming we have a reference element. A large, homogeneously observed stellar sample is ideal for a such a quantitative, statistical study.

Half of the elements heavier than iron were made in the $r$ process. In the $r$-process isotopes are formed very far away from the line of stability in a high neutron flux environment (probably the hot neutrino bubble of an SNII). If a thermal equilibrium situation is reached, the abundances of isotopes depend simply on neutron separation energies (nuclear masses) and half-lives.

We aim at identifying 10-15 new very metal-poor stars strongly enhanced in $r$-process elements, in which thorium and uranium can be detected and which provide nucleo-chronometric age dating. We expect that 2-3 % of the halo stars with [Fe/H]$  < -2.5$ display strong $r$-process enhancement.

Chemically peculiar stars

Chemical Peculiar (CP) stars of the Main Sequence show several characteristics that could be summarized as: i) a non-standard chemical composition of their atmospheres and ii) a variability that involves their luminosity, spectrum and magnetic field.

Chemical Abundances: Carbon. Carbon is one of the major constituents of a normal stellar atmosphere and also one of the principal contributors to the metallicity of the stellar material. It is also a probe to test different theories on diffusion processes involved to explain the abundance anomalies observed in the various classes of CP stars.

The approaches used in literature to study the carbon abundance are sufficiently puzzling. Some authors carried out NLTE analysis to investigate the behavior of carbon lines in standard main sequence stars, some others, on the contrary, based their deduction exclusively on LTE calculations.

To overcome the difficulties arising from theoretical modelling, Catanzaro and Leone analyzed the behavior of carbon in normal and peculiar stars, looking only at the equivalent widths of three important CII lines: $\lambda \lambda$ 4267, 6578 and 6582 Å. The data used for this study have been collected at the CAT-ESO telescope equipped with the CES spectrograph (R=60000) and at the 2.1-m telescope of the Complejo Astrònomico El Leoncito (Argentina) equipped with a Boller and Chivens cassegrain spectrograph (R=12000). The equivalent widths measured in the peculiar stars have been compared with the values measured in the normal stars of the same T$_{\rm eff}$.

The conclusion of this research was that the general trend of the carbon abundance inferred by the equivalent widths of these spectral lines is close to the solar standard value. However, a very large scatter from the normality has been observed in the case of He-weak stars, that could be due to significant abundance differences from the solar value, but no systematic differences have been observed [12].

HD207538. In the attempt to study the chemical composition of stars, a very important task is the modeling of their atmosphere. A good model is a fundamental starting point for a detailed quantitative analysis of the chemical composition. Its importance is stressed in the case of CP stars, for which the overabundances of metals change the structure of their atmospheres.

The most reliable codes presently used for such purposes are the Kurucz's ATLAS and SYNTHE. These programs have been extensively and successfully used to model optical spectra of stars whose spectral types lie between B and A.

With the advent of new instruments from space, new optical windows are opened for researchers. Recently, Far-UV (namely wavelength shortward Ly$\alpha $) high resolution spectra become available thanks to the Far Ultraviolet Satellite Explorer (FUSE).

A new question arises: are the available codes able to reproduce the spectra observed at those wavelengths? To try to give an answer to this question, Catanzaro and Leone, in collaboration with M. Andrè (Institute d'Astrophysique de Paris, France) and P. Sonnentrucker (Johns Hopkins University, Baltimore, MD, USA), modelled the observed spectrum of HD207538 (BO V) from far-UV to visible with a single set of stellar parameters. They selected HD207538 within their program dedicated to the study of magnetic CP stars. An optical spectrum of this star has been obtained with the high resolution spectrograph (SARG) at Telescopio Nazionale Galileo. UV and far-UV spectra have been extracted from the IUE and FUSE archive respectively. ATLAS9 has been used to compute the model of the atmosphere and SYNTHE to reproduce the observed spectrum.

Because of the inter-stellar matter (ISM) present between the star and us, the FUSE spectrum is strongly contamined by ISM spectral lines. Thus, they decided to perform an abundance analysis of the ISM material toward the line of sight of HD207538. The ISM theoretical spectrum has been computed with the code OWENS developed by M. Lemoine and the FUSE French team. This spectrum has been combined with the one computed with SYNTHE and the result has been compared with the observations.

Figure 2.19: Comparison between theoretical and observed spectra in the FUV range $\lambda \lambda$ 1000 - 1098 Å. In each panel we plotted at the top the ISM model (green line) with labelled the identified lines, in the middle the synthetic photospheric model (blue line) with the identified lines and at the bottom the observed spectrum compared with the total model (red line).
\begin{figure*}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/fuse_fit1.ps,width=10cm,angle=-90}}\end{figure*}
Figure 2.20: Comparison between theoretical and observed spectra in the FUV range $\lambda \lambda$ 1098 - 1188 Å. Symbols as in Fig. 2.19.
\begin{figure*}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/fuse_fit2.ps,width=10cm,angle=-90}}\end{figure*}
Figure 2.21: Comparison between theoretical and observed spectra in the IUE range selected ( $\lambda \lambda$ 1380-1670 Å). In each panel we labelled the identified lines. ISM lines have been marked with arrows.
\begin{figure*}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/iue.ps,width=8cm}}\end{figure*}
Figure 2.22: Comparison between theoretical and observed spectra in the visible range $\lambda \lambda$ 4625-4720 Å (top panel) and $\lambda \lambda$ 5650 - 5750 Å (bottom panel). In each panel we labelled the identified lines.
\begin{figure*}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/tng_fit.ps,width=10cm,angle=-90}}\end{figure*}
Figure 2.23: Helium lines observed in the visible spectrum of HD207538. For each line we overplotted the corresponding synthetic profile (solid/red curve). The helium abundance used is log $\epsilon$ = 11.30.
\begin{figure}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/helium9.ps,width=9cm}}\end{figure}

Catanzaro, Leone and collaborators demonstrated that they could reproduce the observed spectrum of HD207538, including also the ISM features, with a single set of atmospheric parameters (T$_{\rm eff}$=32190 K, $\log g$=4.32 and $\xi$=8 km s$^{-1}$) and chemical abundances. Results of their modeling are showed from Figs 2.192.202.212.222.23. As a by-product of their analysis, they ruled out the studied star from the CP group, since any of the observational characteristics of CP stars have not been observed on it. In particular, to check out if the star shows spectral variability, they collected some spectra at the Catania Astrophysical Observatory. No hints of variability have been found (Catanzaro et al. 2003, submitted to A&A).

Stratification of the elements. With the aim of studying the possible vertical stratification of the chemical elements along their atmosphere, Catanzaro, Leone and M. André performed spectroscopic observations of a sample of helium peculiar stars at 1.5-m ESO telescope in La Silla (Chile) equipped with the FEROS spectrograph. Unfortunately, due to bad weather conditions they could get spectra only for 6 out of the 30 stars scheduled. In any case, these high resolution spectra ($R  =  48000$) should be enough to allow a preliminary analysis. This research is still in progress.

Spectral variability in chemically peculiar stars

Helium line variability. To try to understand the behavior of helium variability in CP stars, Catanzaro and Leone continued their on-going observational campaign started two years ago (Catanzaro et al., 1999). This year, they performed new time-resolved spectroscopic observations of the HeI5876 Å line for a sample of 10 stars in the spectral range B3 - A2, characterized by different overabundances. The observations have been performed with the spectrograph of the 91-cm telescope of Catania Astrophysical Observatory.

In the scenario described by the Oblique Rotator Model spectroscopic, photometric and magnetic data should vary with the same period. Since the work of Mihalas (1973), the variability of the strength of helium lines has been explained by the presence of large caps of such element placed around the magnetic poles. In such a way, the rotation of the star is responsible of the observed spectral variability.

The principal aim of this kind of studies is to explore the phase relations among spectral, light and magnetic variations by searching for correlations with physical properties of the stars such as spectral type or chemical peculiarities.

To pursuit their goal, Catanzaro and Leone searched in the literature for photometric and magnetic measurements. Hipparcos photometric catalogue has been queried for light curves. Data for all stars of the sample have been found, while for five of them they found magnetic observations. As an example, the observed spectral variability for HD124224 and HD142990 are reported in Figs. 2.24, and 2.25

Figure 2.24: Equivalent width variations of HD124224 versus phase. Equivalent widths of the HeI$\lambda$4471 Å have been taken from Kuschnig et al. (1999). Magnetic data are plotted with different symbols according to their source: open circles for those after Borra (1980), while filled circles for those after Pyper et al. (1998). Photometry is taken from Hipparcos.
\begin{figure*}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/hd124224.ps,width=8cm}}\end{figure*}
Figure 2.25: Equivalent width variations of HD142990. Photometry is taken from Hipparcos. Magnetic observations after Borra et al. (1983).
\begin{figure*}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/hd142990.ps,width=8cm}}\end{figure*}

Thanks to this new data-set, Catanzaro and Leone explored the possible relations with peculiarity classes and lengths of the rotational periods. They confirmed the conclusions published previously (Catanzaro et al., 1999), that is: no-unique correlation exists and this fact is independent of the T$_{\rm eff}$ of observed stars, the peculiarity classes and the periods of variations. According with the estimated T$_{\rm eff}$ and $\log g$, they classified the program stars with respect to their helium abundance; the results are reported in Fig. 2.26 [38].

Figure 2.26: Behavior of equivalent width versus effective temperature for our sample of CP stars. Points represent the observations, error bars extend by 1$\sigma$. Curves represent the NLTE calculations by Leone & Lanzafame (1998) for three different values of $\log g$: 3.5 (long dash), 4.0 (short dash) and 4.5 (dot).
\begin{figure*}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/growth.ps,width=8cm}}\end{figure*}
 

HgMn in binary systems. Catanzaro and Leone continued their campaign devoted to the study of the spectroscopic binary stars with a chemically peculiar component. To deal with an homogeneous sample, they selected all the systems with a HgMn component. As the name of the class implies, in the HgMn stars lines of both Mn and Hg are abnormally strong. In addition, lines of elements not seen at all in normal stars may be quite prominent, such as those of gallium, platinum, phosphorus, xenon and krypton.

They selected 34 systems in total and for the 27 already observed they could calculate orbital parameters. To address this aim, they wrote an IDL procedure that compute the best orbital solution by minimizing the differences between observed and theoretical points.

From this sample, they extracted all the 5 SB2 systems with the aim of performing an abundance analysis of both components. Presently they are analyzing HD191110 using spectra taken at Catania and extracted from the CFHT archive. To compute atmospheric parameters of each component, they wrote an IDL procedure to fit simultaneously the spectra of each star. As an example, the composite H$\beta $ is reported in Fig. 2.27. This analysis is still in progress.

Figure 2.27: Simultaneous fit of the composite H$\beta $ of HD191110. The observations have been carried out at the 91 cm telescope of the Catania Astrophysical Observatory with $R  =  14000$. With dashed blue and red lines we showed the model adopted for each component, while solid green line represents the composite synthetic spectra.
\begin{figure*}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/hbeta.ps,width=7cm}}\end{figure*}

Measurement of magnetic fields in chemically peculiar stars

Since CP stars are characterised by a periodically variable magnetic field, the monitoring of the Stokes V parameter is a fundamental step to recover the magnetic field topology. Therefore, Leone, Catanzaro and S. Catalano undertook a program to measure the polarization of the light from CP stars (see, e.g., Leone et al. 2000).

From Hipparcos absolute magnitudes, it is possible to determine the stellar radii and then, on the hypothesis of a rigid rotator, the inclination of the rotational axes with respect to the line of sight. On the hypothesis that the magnetic field presents a dominant dipolar component (that is, where the Stokes Q and U parameters are not necessary to recover the magnetic configuration), it is possible to determine the angle between the rotational and dipole axes and the polar strength of the magnetic field. CP stars show periodic anti-phase light variations short-ward and long-ward of a constant wavelength, the null wavelength. Leone and collaborators have performed numerical computations of the expected flux distribution for metal-enhanced atmospheres with different effective temperature and gravity. From the behaviour of the null wavelength, they confirm the importance of the non-homogeneous distribution of elements on the stellar surface as the origin of the light variability. However, to explain the photometric variability of some stars, they suggested that the flux distribution is also influenced by the contribution of the magnetic field to the hydrostatic equilibrium.

In order to improve the polarization measurements and extend the sample of observable stars, Leone, S. Catalano and Catanzaro proposed the development of a spectropolarimetric modulus for the Telescopio Nazionale Galileo, which was subsequently designed and built in collaboration with the technical staff of Catania Observatory and R. Gratton and R. Claudi of Padua Observatory.

This work led to the polarimeter of the high-resolution spectrograph (SARG) of Telescopio Nazionale Galileo which has been presented by Leone et al. [114,115].

Formation and evolution of stars

INAF Researchers: G. Catanzaro, G. Lanzafame, F. Leone, A. Magazzù,
  S. Scuderi
University Researchers: G. Belvedere, R. A. Zappalà
IRA Researcher: G. Trigilio, G. Umana
PhD Students: G. Palazzo, R. G. Pizzone
Fellow: V. Costa

Hot stars in Local Group galaxies

Local Group galaxies are commonly considered as important astrophysical laboratories useful to study the star formation history in environmental conditions different from those in the Milky Way.

In this framework Catanzaro and Scuderi, in collaboration with L. Bianchi (Johns Hopkins University, Baltimore, MD, USA) continued their effort aimed at characterizing the content of two nearby galaxies of the Local Group: M33 and NGC6822. They analyzed new spectroscopic data for eight objects (seven stars and the nucleus) in M33 and three stars in NGC6822. The observations have been performed with the ISIS spectrograph mounted on the 4.2-m William Herschel Telescope at the Observatorio del Roque de los Muchachos (La Palma, Spain).

For all the stars they derived important atmospheric parameters, such as effective temperature and gravity. For those showing clear evidence of stellar emission in H$\alpha $, they estimated also the mass-loss rate. Examples of their results are reported in Fig. 2.28.

Figure 2.28: Normalized spectra of stars observed in M33. The smooth thick lines are the models for the adopted temperatures and gravities with an estimated wind emission (red lines), whereas the dashed (blue) lines refer to the photospheric models calculated before the mass-loss correction.
\begin{figure*}\centerline{
\psfig{file=/home/gle/oacrep02/rep2002/./catanzaro/plot_hot_m33.ps,width=10cm,height=6cm,angle=-90}}\end{figure*}

In summary, in M33 they found 5 hot star candidates out of 7 stars analyzed, and 2 out of 3 in NGC6822. Although the limited data quality does not afford a precise measurement of stellar parameters, their results indicate that the observed fields sample very rich OB associations. Their spectra add new data-points to the census of hot massive stars in these nearby Local Group galaxies [37].

Stellar Populations studies using HST

M. Romaniello, Scuderi, N. Panagia and R. P. Kirshner have developed a new technique based on multi-band near ultraviolet and optical photometry to measure both the stellar intrinsic properties, that is luminosity and effective temperature, and the interstellar dust extinction along the line of sight to hundreds of stars per square arcminute. The yield is twofold. On the one hand, the resulting reddening map has a very high spatial resolution, of the order of a few arcseconds, and can be quite effectively used in regions where the interstellar material is patchy, thus producing considerable differential extinction on small angular scales. On the other hand, combining the photometric information over a wide baseline in wavelength provides an accurate determination of temperature and luminosity for thousands of stars. As a test case, they studied the region around Supernova 1987A in the Large Magellanic Cloud imaged with the WFPC2 on board the Hubble Space Telescope. Figure 2.29 shows the H-R diagram of the stars in the region [23]. Furthermore, they applied this technique to two other fields in the LMC (Romaniello et al., 2003, submitted to A&A).

Figure 2.29: HR diagram for the 13,098 stars detected in the control field for SN1987A. Black dots are the 4,912 stars with $\delta \log (T_e)<0.05$. The open squares indicate stars for which the F300W magnitude is ill-determined because of saturation and, hence, the fit was performed excluding this filter. The theoretical ZAMS is indicated by a solid line.
\begin{figure}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./scuderi/sn87a_hr.ps,width=8cm}}\end{figure}

Star formation in the bulge of M51

H. Lamers and collaborators (including Scuderi) studied the inner kpc of the interacting galaxy M51 in six bands from 2550 Å to 8140 Å using observations with the WFPC2 on board the Hubble Space Telescope. The extinction properties, derived in four fields in and outside dust lanes in the bulge, are similar to the Galactic extinction law. The reddish stellar population has an intrinsic color of $(B-V)_0 \simeq 1.0 $ suggesting an age in excess of 5 Gyrs. They found 30 bright point-like sources within 110 to 350 pc from the nucleus whose spectral energy distributions were compared with those predicted for models of clusters or single stars, supporting the conclusion that the point sources are indeed single stars [16].

Searches for brown dwarfs

Magazzù, together with colleagues at IfA, CFHT, and Grenoble Observatory, has searched for brown dwarfs in a CFHT survey of the Taurus star-forming-region [117]. In particular, infrared spectra of brown dwarf candidates, obtained with NICS at Telescopio Nazionale Galileo, led to the discovery of six new substellar objects [116]. Together with colleagues of Arcetri and Telescopio Nazionale Galileo, a new classification scheme for infrared spectra of very cool objects has been presented [90].

The radio emission of $\beta $ Lyrae

Umana, Leone and Trigilio presented new observational evidence that supports the presence of an extra source of continuum emission in the binary system $\beta $ Lyrae. New VLA and IRAM observations, together with published data from the literature and ISO archive data, allowed them to build the Spectral Energy Distribution of the binary between $5\times 10^{9}$ Hz and $5 \times 10^{15}$ Hz. The radio-millimeter part of the spectrum is consistent with free-free emission from a symbiotic-like wind associated with the primary component and ionized by the radiation field of the hidden companion. Furthermore, they also considered the possibility that the observed radio flux originated from collimated radio structures associated with the mass-gaining component and its disk (conical thermal jets). An extrapolation of this emission to the far-IR part of the spectrum indicates that in both cases the free-free contribution at these frequencies cannot explain the observations and that the observed infrared excess flux is due principally to the secondary component and its associated disk [26].

Nuclear Astrophysics

A group of researchers of Osservatorio Astrofisico di Catania (OAC), Dipartimento di Fisica e Astronomia and Dipartimento di Metodologie Chimiche e Fisiche per l'Ingegneria of the University of Catania, Istituto Nazionale di Fisica Nucleare (INFN) and Laboratori Nazionali del Sud (LNS) have been maintaining a scientific collaboration on topics concerned with the Nuclear Astrophysics research field, and devoted both to experimental and theoretical activities.

Some basic problems arise when facing the study of nucleosynthesis processes in stars or interstellar medium as well as of cosmological nucleosynthesis. Experimental information on the cross sections of the many reactions involved in these processes is quite sparse and difficult to improve. This is due to the low range of energies ($\sim 1-1000$ keV) at which these reactions typically occur. Because of the ``Coulomb barrier'' affecting charged particle induced reactions, cross sections in the $10^{-9} - 10^{-12}  barn$ range are expected. This typically allows only a small number of ``events'' to be detected in laboratories into respect to the background in direct experiments.

For these reasons, cross sections at energies above those typical of astrophysical environments are usually measured and then extrapolations at lower energies are used. This procedure gives rise to significant uncertainties.

The development of indirect methods for cross section measurements is complementary to the approach described above. These are based on the analysis of reactions different from the one under study, which are both easier to study and useful to extract information on the properties of interacting nuclei. The Trojan Horse method is one of these (a given reaction $A(x,y)D$ is studied through measurements made on another reaction $A(c,..)...$ in which $c$ is a ``cluster'' of two particles: $x+s$, where $x$ is the projectile of the reaction under study, while $s$ acts as a ``spectator'' under specific reaction kinematic conditions).

Another experimental obstacle concerns with the many unstable nuclear species involved in nucleosynthesis processes, so that specifically designed facilities to handle radioactive targets and Radioactive Ion Beams (RIB) are required in order to measure reaction cross sections. One of these facilities is currently under development at the Laboratori Nazionali del Sud (INFN) - Catania (Excyt project: www.lns.infn.it/excyt/index.html).

Together with these experimental difficulties there are also problems more strictly connected with the nucleosynthesis process dynamics: a) the ``astrophysical environments'' in which these processes may occur have to be characterized; this requires a connection between the analysis of observational data (both on the physical conditions of these environments and on the abundances of chemical species) and the development of theoretical models; b) the mechanisms responsible for the synthesis of the various nuclear species in a given environment have to be found, and this requires numerical simulation codes for these processes. These codes include a wide range of data on reaction rates, halflives, nuclear masses, nuclear energy levels, spins of many involved nuclear species. We lack experimental measurements on a great part of these data, so that many are obtained through both microscopic and statistical nuclear models.

On the $s$ and $p$ processes. It is known that the binding energy per nucleon of the various stable nuclear species shows a maximum for species with $A\sim 56$, and a ``peak'' on the isotopic composition curve for the solar system is present at these nuclear masses. Moreover, the exo-thermic fusion reaction processes responsible for energy production in stars are not able to account for the production of isotopic species heavier than $^{56}$Fe.

The processes which are believed to be responsible for the synthesis of heavy isotopes ($A> 56$) are typically named with the following letters: $s$ (slow neutron capture), $r$ (rapid neutron capture) and ``p'' (responsible for the production of $p$ isotopes).

The isotopic species classified as ``$p$ isotopes'' are a group of 35 isotopes (from $^{74}$Se up to $^{196}$Hg) on the proton-rich side of the ``stability valley'' of the nuclei chart. Their synthesis cannot be accounted for through the $s$ and $r$ processes. They are believed to take origin through ``photo-erosion'' of neutrons, $\alpha $ particles and protons involving isotopes with $A>
75-80$ and previously formed via the $s$ process. These reactions should occur on the Ne-O layer of massive stars (stars that can develop core-collapse supernova events, with masses greater than about 10 solar masses) during their pre-supernova phase or their explosion as type II supernova.

V. Costa, L. Iapichino and Zappalà examined the physical conditions which are suitable for the development of the $p$ process and investigated also on the impact that the uncertainties on the $s$ process have on the $p$ process outcome. This study has been performed through reaction network simulation codes including about 400 nuclear species for the $s$ process and about 2000 for the $p$ process. After having performed a preliminary study on the physical conditions (temperatures, densities, initial isotopic abundances, process timescales) that are suitable for the $p$ process, the $p$ processe simulation code has been applied to supernova explosion temperature and density profiles of the layer of a 25 $M_{\odot}$ star (initial mass at the ZAMS) obtained from a recent evolutionary stellar model.

Moreover, attention has been concentrated on the large uncertainty of the reaction rate ${\rm ^{22}Ne(\alpha,n)^{25}Mg}$. The underproduction of the $p$ isotopes $^{92}$Mo, $^{94}$Mo, $^{94}$Ru, $^{96}$Ru is a traditional problem of the above described model for the $p$ process in massive stars, and it is mainly due to insufficient production of $s$ nuclei with mass $A \sim 90-130$ during the core He-burning s process, at least when the standard value of the rate for the reaction ${\rm ^{22}Ne(\alpha,n)^{25}Mg}$ is used. If higher values of the rate were used (inside the uncertainty range reported in the literature), it would be possible to partially solve this problem. Therefore, authors they examined of the role played by the reaction ${\rm ^{22}Ne(\alpha, \gamma)^{26}Mg}$, clarify some aspects of the $s$ process neutron balance [101].

Now a new study of the ``weak component'' of the $s$ process is being developed through a new nucleosynthesis code, which includes an algorithm capable of evaluating the truncation error in numerical integrations. This new study is aimed at the analysis of the uncertainties connected with the stellar model, and particularly to convection treatment, through the use of a stellar evolution code which can handle convection via two different approachs.

Accretion disk models

The formation of accretion disks, with dimensions of the involved structures spanning a range from $\sim 10^{5}-10^{6}$ m (binary systems of compact objects) to $\sim 10^{21}$ m (galaxies), is a frequently observed phenomenon in astrophysics. It is triggered and governed by two fundamental physics laws: gravitation and angular momentum conservation, which are responsible for the formation of a plasma disk around central object, while viscosity and radiative emissions determine the loss of energy and angular momentum of orbitating matter and the subsequent fall of the ``accreted'' onto the central object.

It is generally believed that the origin and evolution of many observed astrophysical phenomena such as novae, some classes of supernovae, X and $\gamma$ emissions, AGN (Active Galactic Nuclei) could be linked in some way to the formation of accretion disks. Therefore, the development of fluid dynamic models to understand the basic working features of these structures could be the key for the understanding of a wide range of astrophysical phenomena. In many of these structures, nuclear reactions occur, and they can play an important role both for energy generation and for chemical evolution.

Spiral structures and shocks often appear in astrophysical gas dynamics, particularly in accretion-disc simulations, but they are difficult to follow numerically. Belvedere and G. Lanzafame considered a black hole of $M_{1}=8$ $M_{\odot}$ as a primary and a small secondary of $M_{2} =0.5$ $M_{\odot}$ , following the idea that a low $M_{2}/M_{1}$ ratio would favour spiral structure and possibly spiral shock onset through gas compression induced by a strong gravitational attraction. In the framework of Smoothed Particle Hydrodynamics (SPH), they performed two 2D models and two 3D models. Simulations in either pair are distinguished by the adopted $\gamma$ value (low and high compressibility cases). Indeed, both 2D models reveal the existence of spiral structures in the disc bulk. Furthermore, spiral shocks at the outer disc edge are evident for the high-compressibility 2D model. Spiral structures appear to develop also for the 3D high-compressibility model. Instead, for the 3D low-compressibility model, no clear conclusion can be drawn as the number of particles (and thus the SPH resolution) is too low [3].

G. Lanzafame, V. Costa, Zappalà and Belvedere investigated the inclusion of chemical evolution in SPH models of accrection disks. The SPH technique has been developed to study ``single-component'' fluids and it needs specific changes to handle phenomena like diffusion, mixing, chemical or nuclear reactions, which are typical of multi-component fluids. After a preliminary study on the role of viscosity [113], a study on the technique to be used in order to insert diffusion processes in the SPH code has been carried out, as a basis for the subsequent future insertion of a ``5-10 species'' reaction network [73].

Determination of the physical parameters of close binaries

Frasca, S. Catalano, Marilli and collaborators obtained for RT Lacertae, a magnetically active close binary, high quality radial velocity measurements which allowed them to determine accurate values of its orbital parameters. A steady decrease of the barycentric velocity from 1920 to 2000 has been pointed out and has been discussed in the context of a third body hypothesis [15] (Fig. 2.30).

Orbital parameters and radial velocities of the single-lined active binary FG UMa have been derived from medium resolution spectra obtained with the REOSC spectrograph at 91-cm telescope [137].

Messina, Frasca and S. Catalano continue a photometric and spectroscopic study in collaboration with Isik from Akdeniz University on the chromospherically active triple system DH Leonis [110].

Figure 2.30: The systemic RV of RT Lacertae in four observing seasons is displayed by filled dots. The orbital solution derived by $\dot{\rm I}$banoglu et al. (2001) is shown with a continuous line. The dotted line represents a solution with a larger period and mass of the third body, which fits better the observations.
\begin{figure}\centerline{
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\end{figure}

Search for extra-solar planets

INAF Researchers: G. Bonanno, R. Cosentino, A. F. Lanza, I. Pagano, S. Scuderi
University Researchers: M. Rodonò
 

Since 1995, when the Jupiter-like planet 51 Peg b (Mayor & Queloz 1995) was discovered, identification and study of extrasolar planets are between the main goals of the international astronomical comunity. The final aim will be the discovery of other habitable planets and/or the confirmation of existence of life on them.

Radial velocity exo-planet search. Since year 2000 the SARG team (i.e., the researchers and technicians who partecipated to the design and building of the high-resolution spectrograph for the Telescopio Nazionale Galileo, among whom the Catania technological group, led by G. Bonanno, Cosentino and Scuderi, have been playing a fundamental role) has undertaken a program to search for planets in binary systems [109]. The first result obtained by the team was the discovery that the components of the wide binary HD 219542 differ in metallicity of about 0.1 dex (Gratton et al. 2001). After two years of high-precision radial velocity monitoring of the two components of the binary performed at Telescopio Nazionale Galileo using the high-resolution spectrograph SARG, they found no indication for radial velocity variations above the measurements errors ($\sim$ 5 m/s) for the metal-enriched component. This allows to place limits on the presence of close-in planets around the star [68]. On the other hand, the radial velocity of HD 219542b shows low-amplitude variations with a period 112 days (see Fig. 2.31) at a moderate confidence level. This might suggest the presence of a Saturn-mass planet, although it is still possible that these variations are due to a moderate activity level of the star (Desidera et al. 2003, submitted to A&A).

Figure 2.31: The radial velocity of HD 219542b phased to the 112.1 days period of the best orbital solution.
\begin{figure}\centerline{\psfig{file=/home/gle/oacrep02/rep2002/./scuderi/hd219542b_phase.ps,width=15cm}}\end{figure}

Stellar magnetic activity and exo-planet search. The magnetic activity of solar-like stars, even at the low level characteristic of our Sun, is a potential source of problems in the search for exo-planets. As a matter of fact, it affects the radial velocity measurement accuracy up to a few tens of m s$^{-1}$ as discussed by, e.g., Saar & Donahue (1997) and also the relative variation of the flux integrated over the stellar disk up to a few $10^{-3}$, in the case of active regions having extensions comparable to those of the Sun. This may severely limit the efficiency of detection of Earth-sized planets by means of the transit method, which should be feasible from space by the recently proposed COROT, Kepler and Eddington missions.

The Catania group working on stellar activity has offered his collaboration to the team involved in the exo-planet search for the COROT mission in order to improve the transit detection algorithms. Specifically, Lanza, Rodonò and Pagano, in collaboration with P. Barge and A. Llebaria from Laboratoire d'Astrophysique de Marseille and other people working on the exoplanet transit project have begin to develop methods to reduce the impact of stellar activity on transit detection. Their approach is based on the analysis of the variability of the Sun as a star, for which we have the only set of measurements of accuracy comparable to those of the stellar photometric time series expected to be obtained by the forthcoming space missions. Therefore, the Catania team analysed and modelled the rotational modulation signal present in the time variation of the Total Solar Irradiance (TSI) as observed by the active cavity radiometers of the VIRGO esperiment on board the satellite SoHO. By means of the techniques previously applied to the modelling of light curve sequences of magnetically active stars, they were able to model the most important contribution to the solar flux variability, that is that arising from the growth and decay of the largest active regions whose visibility is modulated by the rotation of the Sun. After subtracting the best fitting model of their contribution, the residual variability is reduced by a factor of $20-30$, which should allow to detect easily the transit of an Earth-sized planet across the disk of a Sun-like star even during period of maximum activity, as it is shown in Fig. 2.32. The preliminary results of the application of the light curve fitting method have been presented in [76], while a full account is in preparation by Lanza et al. (2003).

Figure 2.32: Upper panel:the TSI variation with superposed the central transit of an Earth-like planet the TSI variation with superposed the central transit of an Earth-like planet of radius $R=2.3R_{\oplus}$ and orbital period of 30.0 d around a $1.0$ M$_{\odot}$ star. Lower panel: the same time series after subtracting the best fit model light curve computed with the approach proposed by Lanza et al. (2003).
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Extra-galactic Astrophysics and Cosmology

INAF Researchers: V. Antonuccio, U. Becciani, A. Bonanno, S. Catalano, A. Frasca,
  A. Magazzú, E. Marilli
 

Cosmology: Galaxy Formation

The activities in Cosmology and Galaxy Formation are tightly connected to the activities on Computational Astrophysics. Besides developing N-body codes and visualization and analysis tools, simulations of the evolution of the Large Scale Structure of the Universe are performed, with two main aims:

Figure 2.33: Galaxy structure simulation: Snapshot at the end of the simulation. The run was perfomed using more than 16 million particles. The scale of grey corresponds to density in logarithmic units. The large void is clearly seen in projection extending over the lower left octant.
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The latter activity is particularly useful also for the modelling of the propagation of cosmic rays (and particularly of Ultra High Energy Cosmic Rays).
In aprticular, concerning the first issue, we have used the results of a high resolution N-body simulation to investigate the rôle of the environment on the formation and evolution of galaxy-sized halos. Starting from a set of constrained initial conditions, we have produced a final configuration hosting a double cluster in one octant and a large void extending over two octants of the simulation box. We concentrated our attention on gravitationally bound galaxy-sized halos extracted from the two regions. Exploiting the high mass resolution of our simulation ( $m_{body} = 2.1\times 10^{9} h^{-1} M_{\odot}$), we focussed on halos with a relatively small mass: $5\times 10^{10} \leq M
\leq 2\times 10^{12} M_{\odot}$. We have studied two statistics: the relationship between 1-D velocity dispersion $\sigma_{v}$, and mass $M _{0}$, and the probability distribution of the spin parameter $P(\lambda )$. We have found a clear difference between halos lying in overdense regions and in voids. The $\sigma_{v}-M$, relationship is well described by the Truncated Isothermal Sphere (TIS) model introduced by Ileev, Shapiro and Raga (2001), but the slope of the relationship is larger in voids. We have studied in more detail the TIS model, and we found new relationships between the truncation radius and other structural parameters. After a comparison with the simulation (see Fig 2.33), we have concluded that the structural properties of our halos are well described by the TIS model, although not by the minimum energy solution. We have also shown that the dependence of the statistical properties on environment can be quantitatively accounted for within this model by a different dependence of the dimensionless truncation radius on mass in clustered and void regions.

Cosmology: Universe models

In collaboration, with M. Reuter, Institut fuer Physik, Universitaet Mainz, Germany, and E. Bentivegna, Catania University, A. Bonanno has continued the study of Universe models with dynamically evolving Newton constant $G$ and cosmological constant $\Lambda$ from the renormalization group equation [6,5], by studing a cosmological perturbation theory. E. Bentivegna, A. Bonanno, M. Reuter, using median statistics and a Bayesian model selection criterion have shown that this new cosmology has even better chances of being the correct theory of the late Universe than any standard cosmology. A. Bonanno has also been invited to held a series of lectures at the University of Naples, on the renormalization group and its applications in Physics and Astrophysics. He is collaborating with C. Rubano and G. Esposito on a new lagrangian and hamiltonian formalism for a modified theory of gravity.

Extra-galactic astrophysics: U B V photometry of BL Lac objects, Gamma Ray burts observation

In collaboration with colleagues of ``La Sapienza" Rome University and INAF-Torino Observatory, A. Frasca, E. Marilli and S. Catalano have carried on photometric U B V observations of BL Lac objects, within international programs of short- and long-term multi-band monitoring (WEBT). In particular, results of a multi-site optical campaign in which were collected more than 15000 observation of BL Lac, from May 2000 to January 2001, have been published [28]. The Catania group has given a significant contribution to the determination of the ``intra-day" variation behaviour and of the spectral indices of energy distribution of the source. From the analysis of colour indices, two different mechanism have been proposed to explain the long-term variation and the fast flares phenomena. The first one, essentially achromatic, is mainly based on geometrical effects, i.e. the variation of the relativistic Doppler factor $\delta$ probably due to a change of the viewing angle, while the second one, responsible for the outburst, is strongly wavelength dependent. The spectrum become flatter when the source gets brighter. Variability on time scales as short as 7 hours have been also inferred from autocorrelation analysis [28, 75].

Simultaneous optical and X-ray BeppoSax observations of the two BL Lac objects OJ 287 and MS 1458+22 have been performed in 2001 [44]. These observations have allowed the authors to study the spectral energy distribution (SED), which is different for the two blazars, but can be well reproduced by log-parabolic spectral laws. This law, already observed for the synchrotron emission components in other blazars, can be explained if the emitting particles are accelerated by some statistical mechanism having a probability of energy gain that is a decreasing function of the energy itself.

A. Magazzù has taken part in international observational campaigns aimed at the detection and study of Gamma Ray bursts afterglows, using the Telescopio Nazionale Galileo [45].


Laboratory of experimental astrophysics and Solar System physics

INAF Researchers: G.A. Baratta, G. Leto, M.E. Palumbo, G. Strazzulla
PhD Students: M. Domingo, O. Gomis
Fellowship: G. Ferini
Students: E. Di Stefano
Technical staff: F. Spinella, G. Carbonaro

Background

Solid materials in space, namely ices, silicates and carbons, are present in different environments such as the interstellar medium, comets, asteroids and outer solar system objects. As it is the case for the majority of astrophysical objects, all of the information astronomers have on the physico-chemical properties of solid materials in the universe is obtained by analyzing electromagnetic radiation emitted, absorbed, or reflected from solid objects.

Refractory dust particles (average radius $\sim$0.05 mm) made of silicates or carbonaceous material are released from stars (mainly in the red giant phase) in whose atmospheres they are formed, into the interstellar medium (ISM). Occasionally diffuse clouds in the ISM (n$_H$$\sim$1-10$^3$ cm$^{-3}$, T$\sim$100 K; where n$_H$ is the total number of H atoms, i.e. H+2H$_2$, H$_2$ being the numeric density of the hydrogen molecules) contract to form dense molecular clouds (n$_H$$\geq$10$^4$ cm$^{-3}$, T$\sim$10-20 K). In these regions the numeric density of dust particles (grains) is n$_d \sim$ 10$^{-12}$ n$_H$. The temperature of the dust in dense clouds is as low as 10-20 K and thus virtually all atoms and molecules (with few exceptions such as He and Ne) that impinge on the grains stick on the surface to form ice mantles with an average radius estimated to be of the order of 0.1 $\mu$m. As the cloud contracts, atomic hydrogen is converted into molecular hydrogen, through H+H combination on grains and the consequent release of H$_2$ in the gas phase. This process has important consequences on the chemistry of icy mantles: when H dominates, hydrides species such as H$_2$O, CH$_4$, NH$_3$, CH$_3$OH are expected to form leading to a mantle dominated by polar molecules. When H$_2$ dominates, molecules such as the observed CO and the inferred O$_2$ and N$_2$ accrete on grains to form an outer shell of apolar ices. When a star is observed from behind a dense molecular cloud (field star), its light is absorbed by the matter in the cloud and the analysis of the observed spectrum gives information on the composition of the cloud. While the electromagnetic radiation in the UV-Vis spectral range is completely absorbed, in dense molecular clouds, it is possible to observe in the IR the vibrational absorption spectrum due to the presence of refractories and ices along the line of sight.

Dense molecular clouds, after further contraction, are the places where stars are born. The observation of protostars, (stars still embedded in their placental cloud), is a further probe of the presence of ices in the clouds. In this case the almost blackbody continuum emitted from the young object is absorbed by grains whose temperature changes as a function of the distance from the object. These observations, mainly obtained by IR spectroscopy, may reveal the evolution of ices due to thermal and/or energetic (e.g. interaction with UV photons and/or stellar particle winds and cosmic rays) processing.

Ices are also present on many objects in the Solar System such as the satellites of the external planets (Jupiter and beyond), the planet Pluto, the so called trans-Neptunian objects (a class of numerous small objects not yet well investigated), and comets. In this case, too, the study of the composition of the ices is based on the study of the electromagnetic radiation coming from the Sun and reflected by the surface to the observer.

Energetic (keV-MeV) particles and UV photons impinging on solid surfaces made of refractory (carbonaceous and/or silicates) materials and/or ices are present in a variety of environments in space including the interstellar medium and planetary systems. The study of the effects of ion irradiation and UV photolysis has been based on laboratory simulations of relevant targets bombarded with fast, charged, particles and by Lyman-$\alpha $ photons under physical conditions as similar as possible to the astrophysical ones. Two main effects occur: (1) some material is eroded from the target (sputtering) and (2) some physico-chemical modifications are induced, including the formation of different molecules.

Fast ions penetrating solids deposit energy in the target by elastic interactions with target nuclei and by inelastic collisions causing ionizations and excitations. Thus chemical bonds are broken along the path of the incoming ion and physico-chemical modifications occur, including the formation of molecules originally not present in the target. These molecules include species that can be both more volatile than the parent ones and less volatile. When carbon is an important constituent of the irradiated target it gives rise to a refractory residue which is left over after warming up to room temperature. That residue has a complex structure, and after prolonged irradiation evolves to form hydrogenated amorphous carbon. In the case of UV photolysis, the energy is released to the target material through single photo-dissociations, photo-excitations or ionization events per incoming photon. Also in this case new molecular species are formed.

Different techniques have been used, by different groups, to characterize the chemistry induced by energetic ions and UV photons. In our laboratory we have been using, for about 20 years, in situ IR and Raman spectroscopy.

A number of different ices and mixtures have been irradiated to study their chemical and/or structural evolution. Usually samples are prepared at low temperature (10-20 K) and their spectral characteristic recorded before, during and after processing with energetic ions (3-60 keV) and UV photons (Lyman-$\alpha $, 121.6 nm=10.2 eV). Targets are subsequently warmed-up and spectra are taken at increasing temperatures (20-300 K).

This research, in the past 20 years, has been financially supported by ASI, CNR, Assemblea Regionale Siciliana, CNAA, and MIUR. In year 2002, a research proposal, coordinated by G. Strazzulla and involving also the experimental groups of Napoli (OA-Capodimonte) and Lecce (University) has been submitted and approved by MIUR-Cofin. This proposal titled ``Laboratory studies of silicates present in the Solar System'' is summarized in the section on ``Future studies'' with a closer look to the role of Catania-Lasp group.

Experimental facilities

The Vacuum Chamber

The in situ analyses are performed in a stainless steel high vacuum chamber (see inset in Figure 2.34). Inside the chamber, in which pressure is kept below 10$^{-7}$ mbar, a substrate (crystalline silicon) is placed in thermal contact with a cold finger whose temperature can be varied between 10 K and 300 K. A needle valve is used to admit pre-prepared gases (or mixtures) into the chamber, where they freeze on the substrate. A He-Ne laser can be used to monitor the thickness of the ice film during accretion; this is achieved by looking at the interference pattern (intensity versus time) given by the laser beam reflected at an angle of 45$^{o}$ both by the vacuum-film and film-substrate interfaces. Solid samples are simply mounted in thermal contact with the cold finger.

Figure 2.34: Schematic view of the experimental apparatus used for in situ Raman spectroscopy of ion irradiated samples. The inset gives details of the vacuum chamber. In order to obtain infrared spectra the glass objective is removed. A hole in the sample holder allows the infrared beam to transmit through the substrate and the sample.
\includegraphics[width=160mm]{/home/gle/oacrep02/rep2002/./lasp/framan.eps}

The Ion Implanter

The vacuum chamber is interfaced with an ion implanter (30 kV; Danfysik) from which ions with energy up to 30 keV (60 keV for double ionizations) can be obtained. The ion beam produces a 2$\times$2 cm$^{2}$ spot on the target and current density in the range of 100 nA cm$^{-2}$ to a few $\mu$A cm$^{-2}$. The amount of energy released to the icy samples (dose) is expressed in units of eV/16 amu and is calculated from the knowledge of the ion fluence (ions/cm$^2$), the stopping power (eV cm$^{2}$/molecules) of the chosen projectile, and its penetration depth or range in the target (molecules cm$^{-2}$). The first is given by a current integrator on the path of the ion beam, which measures the charge which reaches the sample during irradiation; the other two parameters are well known and can be provided by software such as SRIM. The penetration depth of 30 keV He$^+$ ions in the icy mixtures studied is about 0.3 $\mu$m. In order to have thicker irradiated samples, and thus spectra with a better signal-to-noise ratio, we irradiate the icy mixtures during deposition. In this case the dose is estimated from the knowledge of the deposition rate (molecules cm$^{-2}$ s$^{-1}$ previously calibrated), the ion flux (ions/cm$^2$ sec) and the energy of impinging ions (eV). When irradiated samples are thicker than the penetration depth of impinging ions, the doses are given in units of ions/cm$^2$. A schematic depiction of three different types of performed experiments is reported in Figure 2.35.

Figure 2.35: Schematic description (not to scale) of three different types of experiments performed. The sketch on the left hand side refers to ion irradiation of icy films thinner than the penetration depth of impinging ions. In this case ions pass through the sample. The middle sketch refers to an icy sample irradiated during deposition. The process stops when the sample is as thick as required. The sketch on the right hand side refers to ion irradiation of a sample much thicker than the penetration depth of impinging ions. In this case, only the uppermost layers of the sample are irradiated. The ice underneath remains unprocessed and ions remain implanted in the sample.
\includegraphics[width=160mm]{/home/gle/oacrep02/rep2002/./lasp/tretipi.eps}

Upgrade of the ion implanter

The final part of the year has been dedicated to the upgrading of the ion implanter. A post-acceleration has been settled in order to have ion beams up to 200 keV in case of single ionization and 400 keV for double ionized ions. The new beam line has two main advantages. First of all the penetration depth (that is the range which ions travel in matter before being stopped) for a given material depends on the energy of impinging ions. Thus it will be possible to irradiate thicker samples. This in turn means that it will be possible to obtain spectra with higher signal to noise ratio and to detect weaker features in the spectra.

The total energy which ions release in the target per unit path length (also referred to as stopping power) is given by the sum of the elastic and anelastic stopping powers. The relative amount of these contributions depends on the mass and energy of impinging ions. Thus the new beam will give us the possibility to span a larger range of different stopping power values allowing us to study those effects which depends on the specific interaction (elastic versus anelastic) of ions with matter. A view of the setup of the upgraded implanter is shown in Figure 2.36.

Figure 2.36: The new setup of the ion implanter of the Experimental Astrophysics Laboratory at Catania Astrophysical Observatory. After the upgrade from 30 kVolts to 200 kVolts the ion implanter is shielded with a conductive wall. All instruments and control systems are placed out of the restricted area where the beam is accelerated.
\includegraphics[width=160mm]{/home/gle/oacrep02/rep2002/./lasp/DSC00001.eps}

The UV lamp

A hydrogen microwave discharge resonance lamp (Opthos Instruments) is interfaced with the vacuum chamber through an MgF$_{2}$ window; from this lamp mainly 10.2 eV ($\lambda$=121.6 nm) photons are obtained. An aluminium light collector is placed at the end of the lamp in order to increase the number of UV photons that reaches the sample. A light detector, placed at the end of the aluminium light collector, is used to measure the UV flux during photolysis. The detector is a platinum wire which gives a current, by the photoelectric effect, proportional to the UV flux. The wire detector has been calibrated using the procedure described in Baratta et al. 2002.

Infrared Spectroscopy

Infrared spectra are obtained by a Fourier Transform Infrared (FTIR) spectrometer (Bruker Equinox 55). The sample holder has a hole, with a diameter of 4.5 mm, which allows the infrared beam to transmit through the substrate and the sample. The infrared beam forms an angle of 45$^{o}$ with both the ion beam and the substrate holder. Thus, spectra can be taken in-situ before, during, and after irradiation, without tilting the sample. Spectra shown in the following are ratioed to the background spectrum, which includes the substrate, and are taken with a resolution of 1 cm$^{-1}$ unless otherwise specified.

Raman Spectroscopy

Figure 2.34 schematically shows the arrangement used to acquire Raman spectra. A continuous multiline Ar-ion laser beam ($\lambda$=514 nm) enters a confocal illuminator perpendicularly to its optical axis, into which it is deflected by a microprism. The confocal optical system is arranged in such a way that any parallel beam incident along the optical axis in the opposite direction of the laser is focused onto the entrance slit of the spectrometer (Triplemate SPEX). By means of two flat mirrors, the laser beam is reflected towards the vacuum chamber, where it is focused on a 40 $\mu$m spot on the sample. The same objective which focuses the laser beam on the sample collects the Raman-scattered light, which reaches the confocal illuminator going back along the same path of the laser beam. This confocal system makes it possible to obtain Raman spectra of samples located several meters away from the spectrometer with negligible performance losses. The substrate holder is mounted at an angle of 45$^{o}$ both with the ion beam and the Ar-ion laser beam. This configuration offers the advantage that spectra can be easily taken in situ, even during irradiation with ions, without tilting the sample.

Results

The main results obtained during this year are here summarized.

Comparison of ion irradiation and UV photolysis of ices

The effects induced by fast ions and UV photons on astrophysical relevant ices have been separately studied in different laboratory for several years. It is well known that both processes induce chemical and structural modifications of the ice sample. However only few laboratories have the capability to study both effects with the same experimental set-up. Baratta et al. have continued their study on the comparison of the effects induced by energetic ions and UV photons on simple ices namely methane (CH$_4$), methanol (CH$_3$OH) and water (H$_2$O) [2,44]. After ion irradiation and UV photolysis of methane and methanol the intensity of the original spectral features decreases and new features appear indicating the formation of other molecular species such as C$_2$H$_6$ and C$_3$H$_8$ in the case of methane, CO and CO$_2$ for methanol. They have found that these effects are comparable on fresh ices (i.e., after a low dose of few eV released by incoming ions per molecule of the target) while are different at high dose. Due to irradiation in both cases the optical constants of the sample change. It is well known that under ion irradiation hydrocarbons evolve towards a polymer-like material and eventually to a refractory residue. The experiments of UV photolysis also show that a refractory residue is eventually formed. However while the energy released by ions is independent of the optical constants of the sample, the refractory residue is opaque to UV photons. Thus at higher doses impinging ions continue to release energy to the sample which is further modified while UV photons are strongly absorbed at increasing smaller depths as photolysis proceeds and cause negligible additional modifications. This conclusions are also supported by a combined processing experiment. An icy sample of pure CH$_4$ has been first processed with UV photons (up to 35 eV/16amu) and then with 30 keV He$^+$ ions (from 37 eV/16amu to 89 eV/16amu). Results indicate that if a sample previously photolysed is ion irradiated a steep decrease (at the beginning even steeper than the case of ion irradiation alone) of CH$_4$ column density is observed [2].

Figure 2.37: Column density of CH$_4$ deduced from 1300 cm$^{-1}$ band (deformation mode) after ion irradiation with 30 keV He$^{+}$ ions (solid circles) and photolisys with Lyman-$\alpha $ photons (10.2 eV; open circles). Triangles refers to a sample which has been first proscessed with UV photons (up to 35 eV/ amu) and then with He$^{+}$ ions. Straight lines have been drawn to guide the eye.
\includegraphics[width=120mm]{/home/gle/oacrep02/rep2002/./lasp/ch4new.eps}

As concerns the effects of ion irradiation and UV photolysis on water ice, Leto and Baratta [44] performed an experimental study on the structural effects induced by Lyman-$\alpha $ photons in crystalline water ice carried out by in situ infrared spectroscopy. They found that, as already observed in the case of processing with energetic ions, Lyman-$\alpha $ photons are able to fully amorphize the crystalline water ice structure after a dose of few eV per molecule.

Ion irradiation of frozen hydrocarbons

The discovery of abundant acetylene (C$_2$H$_2$) and ethane (C$_2$H$_6$), along with methane (CH$_4$) as gas phase species evaporated from the cold nucleus of comet C/1996 Hyakutake, and later towards other comets, raised the interest of astrophysicists towards these species. Furthermore it has been suggested that acetylene and ethane could be present as condesates in Titan's stratosphere and perhaps mixed with abundant frozen nitrogen on Triton and Pluto. Strazzulla et al. and Baratta et al. [25,32] have irradiated frozen C$_2$H$_2$, C$_2$H$_4$, and C$_2$H$_6$ with 30 keV He$^+$ ions at 12 K to investigate the formation of new molecules and the development of a refractory residue. Jovian and Saturnian moons are also bombarded by heavy ions accelerated in the magnetosphere of the planets. Laboratory experiments have shown that when reactive ions (such as H, C, N, O) impinge on frozen samples, new molecular species which include the projectile can be formed. Thick samples (that is thicker than the penetration depth of impinging ions) of acetylene, ethylene and ethane have been irradiated with 15 keV N$^+$ ions. In all cases, among others, new spectral features appear due to N-bearing molecules. This result gives confidence that nitrogen implantation gives rise to CN bonds in all the studied hydrocarbons and enforce the possibility of their detection on the surface of Jovian and Saturnian moons. Recent observations have shown that the surfaces of some objects in the outer solar system, namely Pluto, Triton, and possibly a number of small trans-Neptunian objects, are dominated by frozen nitrogen mixed with small amounts of methane, carbon monoxide and dioxide, and water. Many other, even more complex, molecules could be present as well, although not yet firmly identified. Even if molecular nitrogen is the most abundant species on the surface of Pluto, the presence of regions rich of methane has been suggested. On the other hand the spectrum of Triton is compatible with the presence of regions with water and carbon dioxide spatially segregated. Furthermore, due to planetary climatic cycles, volatile species sublimate and recondense in a complex manner. Previous laboratory work has concentrated on the study of the effects of ion irradiation of pure CH$_4$, and N$_2$ rich mixtures with H$_2$O and CH$_4$ as minor components. Strazzulla et al. and Baratta et al. [25,32] have considered mixtures where water, methane and nitrogen are in comparable amount and that could be relevant to ``simulate'' those superficial patches where segregation is taking place. Laboratory experiments have shown that several molecules containing cyano groups are formed after ion irradiation at low temperature. This findings is of primary relevance because of their role in the development of a very complex chemistry. These molecules could have been delivered by comets on the primitive Earth during the early heavy bombardment. Here, they could have contributed to the development of more complex biogenetic compounds.

C$_{60}$ Fullerene

Recently it has been proposed that C$_{60}$ fullerene may exist in the interstellar medium although its total amount should be low also because its instability towards the action of electromagnetic radiation which can cause its dimerization and oligomerization. Additionally, fullerene in space is exposed to the action of energetic ions. To simulate in the laboratory the effects of ion bombardment Cataldo et al. [10] have bombarded C$_{60}$ fullerene films with 30 keV He$^+$ ions at room temperature in vacuum. The structural changes undergone by C$_{60}$ have been followed by both infrared and Raman spectroscopy. This latter was the most useful tool for this scope. It has been clearly discovered that at low radiation dose C$_{60}$ forms oligomers but at higher doses it is converted into an amorphous carbonaceous matter. Therefore, C$_{60}$ in space, if condensed on the surface of solid grains and exposed to any kind of radiation, cannot survive as such but it is converted to an oligomeric or polymeric form.

Nitrogen condensation on water ice

Most of the studies relative to water diluted in different solid matrices have been performed in the mid-infrared spectral range where fundamental vibrational modes are present. Palumbo and Strazzulla, [47] have perfomed a new experimental study meant to investigate the spectral characterisitcs in the near-infrared of N$_2$:H$_2$O icy mixtures and samples of N$_2$ diffused in water ice. This spectral range typical of combination and overtone modes, is relevant to study objects in the Solar System. As a matter of fact, based on observations in the 0.5-2.5 $\mu$m (20,000-4,000 cm$^{-1}$) spectral range, the surfaces of the planet Pluto, of the major Neptune's satellite Triton, and possibly of a number of small trans-Neptunian objects, seem to be dominated by frozen nitrogen mixed with small amount of methane, carbon monoxide and dioxide, and water. Due to climatic cycles volatile species, such as N$_2$, are expected to sublimate and recondense on the surface while less volatile species, such as H$_2$O, remain segregated on the surface. Although water easily segregates because its vapor pressure is so different from that of the dominant nitrogen, it cannot be excluded that a small quantity of water molecules can be trapped in nitrogen ice. This could be the result both of a recondensation process of nitrogen from the atmosphere on a water-rich surface patch and of the condensation of the dominant nitrogen along with a small amount of water vapor, not yet detected, but likely present, in the tenuous atmosphere of Pluto and Triton. Laboratory experiments have shown that when water is highly diluted in nitrogen a feature at about 5300 cm$^{-1}$ (1.88 $\mu$m) appears. Palumbo and Strazzulla, [47] suggest that this band should be searched for on these objects.

Asteroids photometry

The data collected during many observational campaigns, made at M.G.Fracastoro station of Catania Astrophysical Observatory, have been analyzed to derive aspect and position of asteroids. The axes ratio values and the pole coordinates of 48 asteroids have been published. For these objects almost three light curves, well distributed in longitude over time including many oppositions, were obtained. The value of the rotational period for more than 50 asteroids was also determined and for as many asteroids the known value was corrected or improved [34]. The V light curves and the B-V mean color index of these objects are in press [33]. It was scheduled an observational campaign, devoted to Trojans, NEOs and small size asteroids, that will be made by using the CCD Kodak camera (see 2.5.4.1) attached to the primary focus of the 67/92 Schmidt of Asiago Observatory.

Future studies

``Laboratory studies of silicates present in the Solar System''. The study of the formation and evolution of different classes of solid bodies in our Solar System is one of the most relevant and puzzling subjects of modern planetology. A whole research community is involved, at international level, to tackle a wide variety of open problems concerning the past and present composition of large and small objects. It is quite evident that such a broad area of research (and the so many open questions) requires a combined effort based on laboratory studies, observations and theoretical work, as usually it occurs in many branches of science. Silicates, carbons and ices are the main components the solid objects in the Solar System are made of. The Italian groups at Catania, Lecce, and Napoli have been involved, for several years, in a wide variety of laboratory experiments aimed at ``simulating'' materials and/or processes occurring in space and appropriate to gain information on material properties in a wide set of space environments. The simulations have been focused on the formation of refractory sub-micrometer silicate and carbonaceous dust in the atmosphere of evolved stars; their evolution in the interstellar medium, including amorphous-crystalline transitions and the formation of ice mantles on the refractory cores, under the effects of external agents (UV photons, energetic particles, thermal processing, etc.); their evolution around young forming stars; their properties and endogenous and/or exogenous processing in the very broad set of different objects in the Solar System.

Along two years Catania-Lasp researchers plan to achieve some specific goals relevant to the most important open questions. The general aim of the research is to contribute to a better knowledge of properties of selected silicates of interest to planetology and some of the physical-chemical processes that might have driven their formation/evolution.

In particular the activity will focus on the study of the following selected silicates:

The detailed study of these materials, along the lines described below, will be possible thanks to the long lasting experience of the proposing team in the field of laboratory characterisation of materials of astrophysical relevance and to the wide variety of experimental techniques, instrumentation and tools already available in the groups. The main available analytical techniques that will be used in the study include: UV-Vis-IR reflectance, UV-Vis-IR extinction spectroscopy, Raman and micro-Raman spectroscopy, IR emission spectroscopy, scanning electron microscopy and energy dispersive X-ray analysis. The silicates will be studied both as bulk materials and as particulate in selected size ranges. They will be produced by grinding and sedimentation techniques and by laser ablation under different environmental conditions. After production and characterisation, the samples will be subjected to different processing: annealing, UV and/or energetic particle irradiation at different temperatures, implantation with reactive energetic ions (H, C, N, O, Na), hydration. All the analyses will be repeated after the processing to study the induced effects. UV-Vis-IR diffuse reflectance and Raman spectroscopy will be performed also ``in situ'' during irradiation.

The main role that Catania-Lasp group will play in this research is the study of the spectral properties of the chosen silicates during and after processing with UV photons and energetic ions. UV irradiation and ion bombardment will be performed to investigate ``in situ'', at low pressure and at different temperatures, some key aspects:

Catania Observatory Laboratory for Detectors (COLD)

INAF Researchers: G. Bonanno, R. Cosentino, S. Scuderi
Students: D. Gandolfi
Technical Staff: M. Belluso, P. Bruno, A. Calì, M.C. Timpanaro

 
 
The main activity of this research group consists in the study and realization of image acquisition systems based on two-dimension detectors and optical instrumentation for astronomical applications. Typical products of this activity are "Front-End electronics" for CCD detectors, photon counting systems, acquisition, analysis and reduction of images, electro-optical characterization of detectors for ground and space telescopes. The optimization of the CCD controller is centered both on the use of new technology circuits such as DSP and FPGA and on the attenuation of the readout noise to obtain high signal-to-noise ratios, thus allowing the detection of weak "features", e. g. smaller than 1% of the continuum, in the case of spectrophotometry.

Since 1998 the group has begun a search program dedicated to select new detectors useful for astronomical observations, both from Earth and space. In addition to CCDs three detectors in particular are under investigation: one based on diamond (Cofin. MURST, ASI), one based on MCP coupled to CMOS-APS sensors to obtain photon counting with the highest dynamical count rate (CNAA funding), and one based on avalanche photodiodes (SPAD) .

The main facilities and instruments available to the group include:

The group activities in 2002 have been developed in the following areas:

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CCD controller update
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New technology detectors
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Collaboration with industries
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Support for CCD camera and control systems
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Partecipation to international projects

CCD controller update

CCD controller for TNG

The new generation CCD controller, developed by the Italian Detector Working Group (DWG), is an improvement of the CCD controller in use at TNG. A new interface with the host computer, based on a high-speed link and PCI board, able to sustain high data transfer rate has been designed and built (Cosentino et al. 2003a, Proceedings of workshop on "Scientific detectors for astronomy'', Waimea, Hawaii, in press). The sequencer has been modified in order to improve high-speed clocks and different reading modes. A new analogue board based on a fast ADC's and new signal processing has been designed. The board is able to process four channels simultaneously allowing high acquisition rates.
The new host interface is a PCI based board, equipped with a full-duplex optical link, working at 1.2 GBauds. It allows data and telemetry communication (remote to local) and commands and clocks sending (local to remote). Communication with high-level languages is guaranteed by low level drivers (Windows NT, 2000 and XP) and a DLL. Thanks to the high speed link the phases are generated in the host computer and are rebuilt by the sequencer. All the TTL control signals are generated on the PCI board (far from the CCD head), while the analogue circuits are next to the CCD head. This guarantees a high performance in terms of noise immunity.
In summary, the analogue board allows: four selectable input gains and four selectable bandwidths to work at different readout speeds, the possibility to program the offsets before and after the CDS stage, to adapt the signal to the A/D converter and the choice of different references for the dummy input. The bias generator allows 16 programmable bias voltages with different ranges, divided in four groups. The voltage ranges are: from 15 to 30, from 5 to 15, from -5 to 5 and from -10 to 10 Volts.

New technology detectors

CVD Diamonds detectors

The use of polycrystalline CVD diamond for the detection of ionising radiation (including UV and X-ray) has been widely reported (see for example Pace, Scuderi & Di Benedetto 2000, Diamond and Related Materials, 9, 987). These results highlight the advantages of diamond as a radiation-hard material with exceptional thermo-mechanical properties. The importance of material quality in device performance has also been reported. Because of advances in CVD synthesis technology the electronic properties that are currently achieved in polycrystalline CVD diamond rival some of the best results obtained with single crystal natural diamonds. It is important therefore to attempt to characterize the quality of the material in addition to device performances.
The project has received fundings by MURST in 1998 and by ASI in 1999 and 2001 and is carried out in collaboration with the Dipartimento di Astronomia e Scienza dello Spazio of the University of Florence, the Dipartimento di Meccanica e Materiali of the University of Reggio Calabria and the Dipartimento di Scienze e Tecnologie Fisiche ed Energetiche of the University of Rome (Tor Vergata).
In the past year the team has worked to the development and the characterization of large area (1 cm$^2$) Vacuum-UV (VUV) CVD diamond photodetectors to address the requirements of space missions where pixel and 2-D arrays are used. The quality of the CVD diamond was characterized by photoluminescence and Raman spectroscopy. The performance of these devices in the dark and under illumination was investigated and the results compared to those from small area detectors based on similar material. Planar and transverse electrode configurations were used in order to evaluate the possibility of producing imaging detectors. The spectral analysis of the photocurrent was measured as a function of several functional parameters and experimental conditions. Figure 2.38 shows the measured quantum efficiency for one of such devices.
Figure 2.38: Absolute Quantum Efficiency of a large area CVD diamond device for two different values of the bias voltage
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Intensified Active Pixel Sensor (IAPS)

The activity of development of a new photon counting detector for the astrophysical applications based on a Micro Channel Plate and a CMOS APS sensor (see Figure 2.39) has been going on since 2000 on CNAA funds. During 2002 we have realized a new electronic board with an FPGA XILINX XCV800 which has allowed us to reach a frame rate of 400 Frame/sec [95,125]. The software for the control electronics card written in C++ language and a series of routines writings in IDL language, allow to the user to measure the characteristics of the system in real time. Furthermore, we have investigated the possibility to improve the performances of the system trying to reach the maximum frame rate possible for the APS sensor, that is 500 frames/s, and also to increase the compactness and reduce the weight of the detector. In the next year we will assemble the first prototype of the detector. This means the realization of the mechanic assembly for the Micro Channel Plate and the optic fibers tapered APS. After laboratory evaluation tests we plan to investigate the scientific capabilities of the detector attaching it at the 91cm telescope of Serra La Nave.

Within this activity a request for "patent" has been deposited at the local Patent office (Camera di Commercio di Catania) named: "A photon counting system based on CMOS-APS detector", which claims a count rate much higher (5 to 10 times) than systems presently available.

Figure 2.39: Block diagram of the CMOS-APS photon counting system
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SPAD detector

SPAD is a p-n junction biased above the breakdown. In this operative condition a hole-electron pair, generated in the depletion layer causes an avalanche, measurable as a current. The diode works as a single photon detector with an infinite gain. The current flows in the diode until the drive circuit, called active quenching circuit (AQC), turns off the polarization for a dead time, defined hold off time, and then restores it. In this way the SPAD is ready for another event. This turn off time depends on the quality of the SPAD.
This activity is carried out since 2001 in collaboration with R&D office of ST microelectronics (STM). During the past year we have measured the characteristics of several devices produced by STM and in particular, quantum efficiency, dark counts, linearity and after pulsing (Cosentino et al. 2003b, Proceedings of workshop on "Scientific detectors for astronomy'', Waimea, Hawaii, in press). We found that the quantum efficiency, measured in the the range 350-1050 nm, is typical of a silicon based detector and shows a peak of about 60% at 550 nm for 100$\mu$m diameter devices (Di Franco et al. 2002, Technical Report ST Microelectronics). Also, as shown in Figure 2.40, it depends on the polarization voltages. Dark counts increase with the bias voltage because the higher electric field increases the probability of avalanche and the depletion volume. After pulsing depends on the following phenomenon: during the avalanche, some carriers can be trapped in the depletion layer and released after a time delay. If this time delay is greater than the hold-off time, the carrier generates an avalanche named "after pulsing". To reduce the after pulsing effect, the hold-off time had to be increased although this reduces the count rate. Both causes of dark counts depend on the defects in the depletion volume and can be reduced with a cleaner fabrication process. Dark counts are measured at 20$^\circ$C, with a hold-off time of 360 ns that reduces the after pulsing effect. Furthermore, during the current year we have designed and realized a new active quenching circuit with improved performances which will be tested next year. For the next year we also plan to characterize two arrays of 5 X 5 elements, with pixel size 20 and 40 $\mu$m in diameter, developed by ST Microelectronics.
Figure 2.40: Quantum Efficiency of SPAD devices with diameter increasing from 10 to 100$\mu$m
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Collaboration with Industries

As the development of new technology is a fundamental activity of the group the interaction with industries is strategic for the fulfillment of this objective. During 2002 the group has started and continued collaboration with three main industries:

Support for CCD Cameras and control systems

The activity of the group for the support and improuvement of the OACt CCD cameras has continued as follows:


The CCD camera for the Schmidt telescope at Serra La Nave

COLD reseachers have designed and realized the electronics and the software for the management of the filter wheel of the CCD camera. The camera will now go through a phase of commissioning at the telescope. Very preliminary test for the use of a cryo tiger cooling system have been made for the CCD camera of the spectrograph.
Figure 2.41: Response of the near UV multilayers filters for the UVISS wide field imaging camera
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Support to solar and stellar observations

The group is involved in the improvement of the performances of the instrumentation for solar and stellar scientific observations. In particular we are in charge for the maintenance of hardware and software of CCD cameras and controllers.

Support to the Schmidt Telescope at Cima Ekar

We have collaborated in the project managed by the Osservatorio Astrofisico di Padova to make the Schmidt Telescope 92/67 at Cima Ekar fully automatic. In particular P. Bruno has developed the user interface for the pointing and the tracking of the telescope and also the software to manage the focus mechanism and the filter wheel of the CCD camera. The telescope will be used to study asteroids and object with orbits close to Earth orbit (NEOs, NEAs etc) and to search for extrasolar planets through the transit technique.

International project collaboration

UV Italian Sky Surveyour. UVISS is a small UV telescope designed for accomodation on the International Space Station (Bernacca et al. 2000, SPIE Proc., 4139, 199). Our team is involved, together with the Osservatorio Astronomico di Brera-Merate and the Istituto di Fisica Cosmica of CNR, in the realization of a wide field camera for near UV (130-260 nm) and, possibly, far UV (90-115 nm) imaging. Critical components of the camera are the filters to select the band-passes of interest. The proposed solution will make use of filters based on the multilayers technique (Scuderi et al. 2000, SPIE Proc., 4139, 223). During the past year the team has worked on the optical design of the filters (see Figure 2.41) and has performed few experimental tests on selected coatings. The results of this work (Conconi et al. 2003, SPIE Proc., in press) are encouraging since they show that the technique is reliable.

X-SHOOTER. The instrument consists of three separate spectrograph arms for the UV-Blue, Visual-Red, and near-IR part of the spectrum with a resolution of about 10,000. X-SHOOTER will be built by an European consortium comprising ESO, Denmark, Netherlands and Italy. Within the Work Package on Detector systems, the Catania group, due to its recognized expertise in the area of detectors and associated electronics will cooperate with ESO in the characterization of the optical detector systems.

PEPSI. PEPSI is a collaboration of the Astrophysical Institute of Potsdam and of the Italian Consortium. PEPSI at the LBT will allow investigating the role of magnetic fields in astrophysics and will be unique among the instruments currently available at 8-10m class telescopes both for its polarimetric capabilities and for the extremely high resolution. The Catania group will be responsible for the electronics and control software and for the characterization of the CCD detector for PEPSI. Moreover cooperation on the polarimeter concept and reduction software is foreseen.

Computational technologies for astrophysics

INAF Researchers: V. Antonuccio, U. Becciani
Post Doc.: A. Germaná
PhD. Students: D. Ferro, A. Romeo
Technical Staff: A. Costa, E. Martinetti

Introduction

The development of high performance computing gives a fundamental improvement to the study of the origin and the evolution of the Universe. This leads to the study of complex scientific problems, generally using parallel algorithms on massively parallel systems MPP and SMP systems. However, the availability of these resources produces a large amount of data that the research needs to manage and analyze. At present, computational resources needed to run large simulations are available only either in high performance computing sites or using distributed resources. Catania Astrophysical Observatory has an SMP system, the IBM SP 9076, with 24 processors and 48 Gbyte RAM memory (Fig. 2.42) principally used for cosmological simulations. This system enables to run very large parallel programs and some parallel codes have been developed by researchers. The Italian Ministry Istruzione e Ricerca Scientifica (MIUR) has recently financed with 170.000 Euros the project Supercalcolo e Visualizzazione Scientifica: problematiche astrofisiche attuali e alta formazione that will allow the OACt to acquire a new IBM Power4 system that will duplicate the present computational power and will give more than one Tera-Byte of available disk space for data storage and analysis. The problem of the data analysis is also a fundamental task of this group, that is involved in a European project for data analysis using the scientific visualization. The tools AstroMD is a public domain code that the OACt is developing together with the CINECA VISit Laboratory, using the Virtual Theatre at the Cineca. AstroMD manages multidimensional data from cosmological simulations, but it can also manage data from observational surveys. The study and the development of gravitational N-body codes using parallel computing techniques is in progress. The tree code has been implemented on three platforms (T3E, Origin and SP) using the IBM SP system acquired by Catania Astrophysical Observatory. Its final version FLY (Fast Leveled tree n-bodyY code) (http://www.ct.astro.it/fly/) has been made accessible to the public (open source) and till now is used by more then 50 researchers all over the world. The project of data analysis proposed by the Catania Astrophysical Observatory and by CINECA for the years 2001 and 2002 was financed by CNAA. It will consist in testing some useful techniques and in developing a package of scientific visualization AstroMD (http://www.cineca.it/astromd) whose first open source version is already available. The package is based on the Visualization Toolkit (Kitware), which is a software object-oriented to 3D graphics. AstroMD performs the multi-dimensional and multi-varied analysis, contains some tools that can visualize clusters, computes specific quantities (i.e. Correlation functions, Minkowski functionals etc..), analyzes vector fields and uses the IVR (Immersive Virtual Reality) techniques. The results have been recently presented at the Europhysics Conference on Computational Physics. The development of AstroMD has been further financed (Sept. 2001 ) by the European Community which has approved the Cosmo.Lab project [107] (http://cosmolab.cineca.it). The results have been presented at several conferences. During the year 2002, in collaboration with University La Sapienza of Rome and ENEA Casaccia, we start the activity Astrocomp, a portal for astrophysical simulations on a grid of Supercomputers financed by the CNR Agenzia 2000 (http://www.astrocomp.it). The idea at the basis of our project is to create a national and European portal which allows to create a repository of easily usable computational codes and a common data base to be made available to the entire national (and international) community. The original characteristic of the project is that, thanks to the synergy among the different research units participating to the project, it will allow to share a large, distributed virtual computational resource assembled by the different platforms made available. More details on these projects are reported in the following paragraphs.

Figure 2.42: IBM SP 9076.
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FLY: a parallel tree-code for cosmological N-Body simulations

FLY is a parallel collision-less N-body code which relies on the hierarchical oct-tree domain decomposition introduced by J. Barnes & P. Hut (1986, Nature. 324, 446) for the calculation of the gravitational force. Although there exist different publicly available parallel treecodes, FLY differs from them because it heavily relies on two parallel programming concepts: shared memory and one-sided communications. Both of these concepts are implemented in the SHMEM library of the UNICOS operating system on the CRAY T3E and SGI Origin computing systems. On systems like the IBM SP where these libraries are not available FLY has been modified to use the local libraries. A more detailed treatment of the parallel computing techniques which have been adopted and of the resulting performances on different systems can be found elsewhere.
Being an open source project, FLY can and must be modified to suit the particular needs of individual users. FLY is a dynamically load balanced code based on four main characteristics: it adopts a simple domain decomposition, a grouping strategy and a data buffering that minimize data communication.
The domain decomposition is based on a fixed distribution of the particles among the processors: the same number of particles is assigned to each processor. The data structures of both particles and tree are subdivided among the PEs to ensure a good initial distribution of the load and to avoid any bottleneck while accessing remote data.
FLY uses a grouping strategy with the aim to compute a component of the force to be applied to all particles inside a grouping cell and to reduce the number of remote access to build the global force on each particle.
With the data buffering, FLY uses the free RAM portion of each PE to allocate dynamically the tree and the bodies data structures in order to cache remote elements.
All this features make FLY able to run very large cosmological simulations on parallel systems with high performances: more than $20 \cdot 10^{4}$ particles per second were computed using 16 PEs on an IBM SP Power4 1300 MHz at Cineca.

Figure 2.43: Main FLY window.
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FLY has a graphical Tcl/Tk interface that help the user to create all the parameter files, excluding the initial condition file.
The main window sets the working directory, the executable directory and will create the not existing directories. Figure 2.43 shows the main window and the window to set some parameters.
FLY will be soon included in the Computer Physic Communications Program Library [31].

Scientific Visualization

In cosmological simulations, one tries to reproduce the formation, evolution and the properties of large scale structures of the Universe. Depending on the scale, one can either study the properties of single objects, like cluster of galaxies or filaments, or analyse the statistical properties of the various structures, like the mass function or the correlation function. These results are finally compared to observations, giving precise indications on the reliability of the simulated model. Simulated structures generally do not show a well defined shape or particular symmetries, and they tend to be irregularly distributed within the computational domain. They analysis requires a powerful 3D representation allowing fast navigation throughout the structures, selection and zoom capabilities, and also the possibility of increasing the resolution and accuracy within user-selected regions.
Figure 2.44: The main GUI of AstroMD allows to visualize data (Render Window), to define variables and projects, to control variables and filters and to save results. The side of the cubic box is 50 Mpc wide. The sample consists of 15000 particles extracted from a N-body simulation of 16 millions of particles at redshift step z=0, showing the formation of several structures .
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AstroMD [129] provides an effective intuitive way of managing and analysing the large amount of data produced by numerical simulations as the results from the N-body tree code [31] which allows to follow the dynamics astrophysical structures in different cosmological frameworks. It can find structures having a not well defined shape or symmetries, and performs quantitative calculations on a selected region or structure.
Figure 2.45: Formation of clusters of galaxies in the universe, with overdensity of 200 with respect to the background density, visualized by isosurfaces, during the evolution of a sample of 15000 particles in a cubic box of 50 Mpc.
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The whole set of particles can be visualized but it's also possible to use a sub-sample of the data, in order to get a faster and easier visualization. It was implemented a procedure which select randomly the sub-sample of data. Specific care has been devoted to avoid systematic errors in the selection procedure, so that the sample is still statistically significant. Different time frames can be shown in a sequence. When particle-representation is used, the position of particles are interpolated between one time-step and the following. This improves the quality of the "animation" giving a fluidity otherwise unachievable. Both the single images and the whole sequence can be saved in bitmap or jpeg format to prepare an animation of the evolution. Enabling the steady-cam, the system can be rotated in Azimuth and Elevation during its evolution. Zoom-in and zoom-out possibilities are also offered. The available prototype of AstroMD is being extended with functionalities for the calculation of quantities of cosmological interest. The most common data format used in Cosmological simulation, i.e. Tipsy, was included in this software. The basic data structure is an array of particle structures in three separate arrays for each of the types of particle used in the simulations: collisionless particles, SPH particles, star particles and their characteristic properties, as potential energy and temperature. Binary and ASCII files can be read. While the Universe may be roughly homogeneous and isotropic on the scale of our horizon, as required by the Cosmological Principle, the distribution of galaxies in space is decidedly inhomogeneous on scales smaller than this. In order to quantify the lumpiness of the matter distribution revealed by redshift surveys and to relate this to models of structure formation, cosmologists employ a variety of statistical tools, the most common of which is the Power Spectrum. It is defined, in the mathematical language of Fourier series, in terms of the fluctuation field of matter density. Automatic structure detection and morphology with user-selectable scale and thresholds, to study the mass density field associated with the particle distribution, was inserted. It distributes the mass of each particle over a regular computational mesh, by using the Cloud-in-Cell smoothing algorithm. Gravitational field calculation, solving the Poisson equation, was implemented too. Moreover, computation of the Power Spectrum of density fluctuations, as plane waves which evolves independently in linear regime, was implemented. It was inserted the associated two-point Correlation Function too, which is its Fourier Transform, using HAM as his function estimator, because it is preferable at large scale, for samples with non uniform density. The clustered zone is inserted in the sampler in which they are. The plot 2D are shown in another window and is updated when the user points out to another clustered zone with mouse.
Figure 2.46: The Power Spectrum of a spherical sub-sample of a whole set of points from a LCDM model simulation at z=0.
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Figure 2.47: Calculation of the Correlation Function of the same model.
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The Minkowski Functionals describe the Geometry, the Curvature and the Topology of a point set. They provide a complete family of morphological measures, which are "robust" for small samples and independent of statistical assumptions of the points set. In fact, considering the set of points in 3D space, supplied by galaxies of a cluster of galaxies, and decorating each point with a ball of radius r, the tool measures the size, shape and connectivity of the spatial pattern formed by the union set of these balls. These characteristics change with the radius r, which may be employed as a diagnostic parameter.
Figure 2.48: Minkowski Functionals of a LCDM simulation.
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It was inserted a group finding algorithm, known as Friends-of-Friends, that involves searching around each sample galaxy for companions close in distance. A particle belongs to a friends-of-friends group if it is within some linking length of any other particle in the group. User must set two parameters: the maximum distance among particles forming clustering and numMembers, the minimum number of clustered particles. At last, FoF cancels all groups whose members are less than numMembers. It was implemented the calculation of the centre of mass of each group, the number of components of groups and the radius of each group. The graphical output contains the grouped particles. The centre of mass of each group identified by ball with radius as clustered group radius. In another window is displayed the plot of the fraction of grouped particles versus the number of components of the groups. When the parameters are changed by user, FoF updates all its outputs.

Grid Computing

Astrocomp is a project developed by the Astrophysical Observatory of Catania, University of Roma La Sapienza and ENEA, and financed by the CNR.
The project goal is to build a web-based user-friendly interface which allows the international community to run some parallel codes on a set of high performance computing (HPC) resources. There' s no need for specific knowledge about Unix commands and Operating Systems. Astrocomp makes some CPU times, on large parallel platform, available to the referenced user .

Figure 2.49: The Astrocomp Portal (http://www.astrocomp.it).
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Astrocomp is a portal which creates a repository of easily usable computational codes and a common database available to the community. The Astrocomp server is based on a PHP-MySQL environment Choi, W. et al. (PHP 4 Developper Guide, 2000, Ulrico Hoepli Editor). Registered users have a grant for HPC systems available to the portal and can run all the astrophysical codes of the portal. They can easily prepare jobs and submit them remotely. The portal allows the user to know in real time the job status and hardware information of an HPC system like CPU usage, memory and queue status. Astrocomp looks like a sole interface for users wishing to run parallel codes included in the portal; it hides HPC complexity creating a common user level.
Astrocomp runs on Apache-Advanced Extranet Server Eilebrecht, L. et al. (Apache Web Server, 2002, MITP-Verlag GmbH, German): this allows you to set up connections with enormous numbers of authorized users, without bogging down the server.
The authorized user can access to his area. Astrocomp, who can receive real time information like CPU and memory usage from hardware resources, builds on demand report pages through PHP code. The user can take a look to the hardware status and choose an available or free platform. The PHP code allows us to build easy-to-read report pages. The user can choose a code and a platform to run his job, he will compile some forms specifying the parameters and all the variables involved in the job. The complete job history for each user is stored in the MySQL database, so he can easily find his past parameters collections and simply modify them. After that Astroadmin, who owns all the sources codes, builds the input files and shell scripts, generates boundary conditions in accordance to the particular code and platform, compiles and submits remotely the source code. After job completion the PHP code allows the user to obtain a graphical representation of output data (this is useful for a quick look analysis); after that he can download every output file of his job. A constant management of software packets and the PHP code versus most cost common security bugs and attacks like buffer overflows and common exploits is actually one of the goals of the project.
The goal of the project is to manage a grid of HPC. We will use an infrastructure like Globus in order to create a middleware over the hardware resources. The Globus Project is a research and development project focused on enabling the application of Grid concepts to scientific and engineering computing. The Grid refers to an infrastructure that enables the integrated, collaborative use of high-end computers, networks, databases, and scientific instruments owned and managed by multiple organizations. A Grid provides some abstractions and concepts that let applications access across distributed area networks.

Conclusion

The Astrophysical Observatory of Catania will continue and expand all the above mentioned activities also using the new financed project of the MIUR.
The Scientific Visualization will be enhanced also considering some aspects related to low cost new technologies for immersive visualization; the FLY project will include new features for the integration of the code with hydrodynamical codes like FLASH. Grid-based projects will be also developed jointly to CINECA for the extension of Astrocomp functionalities as a Globus-based portal. OACt MPP computing systems will be linked to the INFN Grid, with an agreement actually in progress, that will allow the OACt to include computing facilities in the INFN Grid.


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Giuseppe Leto 2003-06-27