|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,|
|PhD Students:||L. Contarino, A. Murabito, P. Romano., S. Di Giorgio|
More specifically, research activities have been conducted according to the following scheme.
The systematic observations of the Sun in white light and H 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:
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 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 .
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 (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 and 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 .
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 :
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 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 and 6.7, where 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 .
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 , and 171 images obtained by TRACE, with BBSO 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 .
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 of turns of a magnetic field line over the whole length of the prominence was , while the value of the ratio between the pitch of the magnetic field lines and the prominence width was . 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 to 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 data acquired by THEMIS operating in IPM mode, data and magnetograms obtained at the Big Bear Solar Observatory, MDI magnetograms and images taken by TRACE. Observations at show that 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 line is observed to take place hours before the flare peak, while one thread of the filament erupts 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].
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 () 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 .
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.
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 consistent within errors with the earlier results. By considering the OPAL equation of state they obtain a value of 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
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 -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 .
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.
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:
|INAF Researchers:||I. Busà, S. Catalano, G. Catanzaro, G. Cutispoto, A. Frasca,|
|A. F. Lanza, F. Leone, G. Leto, E. Marilli, S. Messina,|
|University Researchers:||G. Belvedere, A. C. Lanzafame, M. Rodonò|
|IRA Researchers:||C. Buemi, P. Leto, C. Trigilio, G. Umana|
|PhD Students:||K. Biazzo|
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 and 6.7, where 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 .
Pagano, in collaboration with J. Linsky of JILA and University of Colorado and other colleagues has obtained and analysed high resolution echelle spectra of 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 .
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, H, HeI D, NaI D,D 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, CaII H & K, H and HeI D and a strong filling-in of the NaI D,D 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 and HeI D 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 .
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 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 , 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:
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 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, performed in summer 2000 on the system RT Lac, rotational modulation of the H emission has been detected in both components . However, while the H 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 . The net H equivalent width, arising from both components, shows a phase-dependent variation. By deconvolving the H 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 emission, interpreted as due to chromospheric plages.
Results on the systematic annual H and photometric observations obtained at Catania Astrophysical Observatory from 1989 to 1997 for HK Lac have been reported. H excess-emission, evaluated with the spectral synthesis method, appear in all cases anti-correlated with the band light curves. From model solutions of light curve obtained in the hypothesis of circular spots the usual parameters , , and for the dark spots have been derived, where and are the latitude and the longitude of the center of the spot of radius , respectively. For modelling the H emission equivalent width () curve, bright spots were considered, assuming the emission flux ratio ( ) between plage and quiet chromosphere as a free parameter .
Longitude differences ( ) between spot and plages do show a systematic change from the most positive value of +40 in 1989 to the most negative value of 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.
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.
|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|
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: . 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 , 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 . 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 .
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  .
"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 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  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 130 Myr and 700 Myr: EK Dra, UMa, HN Peg, k Cet and BE Cet. Also they include in this study the Pleiades-age ( 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 ( ) with age and Rossby number () 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)
- 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).
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 . 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) 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 . 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 The well-established correlation between X-ray luminosity and rotation rate is clearly observed. All single unevolved solar-type stars with larger than 18 km s 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].
Paternò, Belvedere, Kuzanyan (IZMIRAN, Solar-Terrestrial Dept., Moscow) and Lanza investigated the behaviour of a simple thin-shell 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 ( times the solar one), while they tend to be saturated for stars with high rotation rates ( 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 to the 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 .
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 -dynamos which are known as nonoscillating. With the known anisotropic -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 -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 G is needed in order to produce a period modulation of order which has been observed .
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 .
|INAF Researchers:||A. Bonanno, M. P. Di Mauro, A. Frasca, A. F. Lanza,|
|University Researchers:||L. Paternó, M. Rodonò|
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 .
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 Bootis and of Hydri.
The subgiant HR 5235, better known as 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 and a small frequency separation .
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 and are consistent with observations (filled symbols). The model frequencies for modes with are strongly affected by avoided crossings and they deviate from the expected asymptotical behaviour. From the study of Di Mauro et al. , it can be concluded that the observations indicate two possible main evolutionary scenarios for 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 ).
The other target considered is HR2021, better known as 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. 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. 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 , the age is about Gyr and the radius is . The theoretical models can reasonably reproduce the observed spectrum. In particular, Di Mauro and collaborators found that the theoretical large separation is about , while the theoretical small separation is about , values which are certainly consistent with the observed ones. However, while the evolutionary scenario of 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 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 Scuti stars, namely V480 Tau and Tau A, both members of the Hyades open cluster [58,103].
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 K and . 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 s. This star seems to be well-suited for high-precision measurements, which could detect a possible multi-mode pulsation behaviour .
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 , whereas structural and surface effects controlled by the magnetic fields are expected to be dependent. Therefore, Lanza and Rodonò proposed a method that correlates the variation of the 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 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 .
|INAF Researchers:||I. Busà, S. Catalano, G. Catanzaro, F. Leone|
|University Researchers:||F. A. Catalano|
|IRA Researchers:||P. Leto|
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 process. In the -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 -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] display strong -process enhancement.
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: 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.
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 .
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) 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.
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=32190 K, =4.32 and
=8 km s) and chemical abundances.
Results of their modeling are showed from Figs 2.19, 2.20, 2.21, 2.22, 2.23.
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 () should be enough to allow a preliminary analysis. This research is still in progress.
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
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 of observed stars, the peculiarity classes and the periods of variations. According with the estimated T and , they classified the program stars with respect to their helium abundance; the results are reported in Fig. 2.26 .
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 is reported in Fig. 2.27. This analysis is still in progress.
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].
|INAF Researchers:||G. Catanzaro, G. Lanzafame, F. Leone, A. Magazzù,|
|University Researchers:||G. Belvedere, R. A. Zappalà|
|IRA Researcher:||G. Trigilio, G. Umana|
|PhD Students:||G. Palazzo, R. G. Pizzone|
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, they estimated also the mass-loss rate. Examples of their results are reported in Fig. 2.28.
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 .
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 . Furthermore, they applied this technique to two other fields in the LMC (Romaniello et al., 2003, submitted to A&A).
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 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 .
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 ( keV) at which these reactions typically occur. Because of the ``Coulomb barrier'' affecting charged particle induced reactions, cross sections in the 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 is studied through measurements made on another reaction in which is a ``cluster'' of two particles: , where is the projectile of the reaction under study, while 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
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
spins of many involved nuclear species.
We lack experimental measurements on a great part of these data, so that many are obtained through
microscopic and statistical nuclear models.
On the and processes. It is known that the binding energy per nucleon of the various stable nuclear species shows a maximum for species with , 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 Fe.
The processes which are believed to be responsible for the synthesis of heavy isotopes () are typically named with the following letters: (slow neutron capture), (rapid neutron capture) and ``p'' (responsible for the production of isotopes).
The isotopic species classified as `` isotopes'' are a group of 35 isotopes (from Se up to Hg) on the proton-rich side of the ``stability valley'' of the nuclei chart. Their synthesis cannot be accounted for through the and processes. They are believed to take origin through ``photo-erosion'' of neutrons, particles and protons involving isotopes with and previously formed via the 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 process and investigated also on the impact that the uncertainties on the process have on the process outcome. This study has been performed through reaction network simulation codes including about 400 nuclear species for the process and about 2000 for the process. After having performed a preliminary study on the physical conditions (temperatures, densities, initial isotopic abundances, process timescales) that are suitable for the process, the processe simulation code has been applied to supernova explosion temperature and density profiles of the layer of a 25 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 . The underproduction of the isotopes Mo, Mo, Ru, Ru is a traditional problem of the above described model for the process in massive stars, and it is mainly due to insufficient production of nuclei with mass during the core He-burning s process, at least when the standard value of the rate for the reaction 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 , clarify some aspects of the process neutron balance .
Now a new study of the ``weak component'' of the 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.
The formation of accretion disks, with dimensions of the involved structures spanning a range from m (binary systems of compact objects) to 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 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 as a primary and a small secondary of , following the idea that a low 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 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 .
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 , 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 .
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  (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 .
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 .
|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 . 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 ( 5 m/s) for the metal-enriched component. This allows to place limits on the presence of close-in planets around the star . 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).
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 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 , 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 , 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 , while a full account is in preparation by Lanza et al. (2003).
|INAF Researchers:||V. Antonuccio, U. Becciani, A. Bonanno, S. Catalano, A. Frasca,|
|A. Magazzú, E. Marilli|
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 ( ), we focussed on halos with a relatively small mass: . We have studied two statistics: the relationship between 1-D velocity dispersion , and mass , and the probability distribution of the spin parameter . We have found a clear difference between halos lying in overdense regions and in voids. The , 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.
Simultaneous optical and X-ray BeppoSax observations of the two BL Lac objects OJ 287 and MS 1458+22 have been performed in 2001 . 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 .
|INAF Researchers:||G.A. Baratta, G. Leto, M.E. Palumbo, G. Strazzulla|
|PhD Students:||M. Domingo, O. Gomis|
|Students:||E. Di Stefano|
|Technical staff:||F. Spinella, G. Carbonaro|
Refractory dust particles (average radius 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 (n1-10 cm, T100 K; where n is the total number of H atoms, i.e. H+2H, H being the numeric density of the hydrogen molecules) contract to form dense molecular clouds (n10 cm, T10-20 K). In these regions the numeric density of dust particles (grains) is n 10 n. 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 m. As the cloud contracts, atomic hydrogen is converted into molecular hydrogen, through H+H combination on grains and the consequent release of H in the gas phase. This process has important consequences on the chemistry of icy mantles: when H dominates, hydrides species such as HO, CH, NH, CHOH are expected to form leading to a mantle dominated by polar molecules. When H dominates, molecules such as the observed CO and the inferred O and N 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- 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-, 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.
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 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 both by the vacuum-film and film-substrate interfaces. Solid samples are simply mounted in thermal contact with the cold finger.
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 22 cm spot on the target and current density in the range of 100 nA cm to a few A cm. 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), the stopping power (eV cm/molecules) of the chosen projectile, and its penetration depth or range in the target (molecules cm). 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 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 s previously calibrated), the ion flux (ions/cm 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. A schematic depiction of three different types of performed experiments is reported in Figure 2.35.
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.
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 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 unless otherwise specified.
Figure 2.34 schematically shows the arrangement used to acquire Raman spectra. A continuous multiline Ar-ion laser beam (=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 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 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.
The main results obtained during this year are here summarized.
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), methanol (CHOH) and water (HO) [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 CH and CH in the case of methane, CO and CO 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 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 column density is observed .
As concerns the effects of ion irradiation and UV photolysis on water ice, Leto and Baratta  performed an experimental study on the structural effects induced by Lyman- 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- photons are able to fully amorphize the crystalline water ice structure after a dose of few eV per molecule.
``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:
|INAF Researchers:||G. Bonanno, R. Cosentino, S. Scuderi|
|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:
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.
The activity of the group for the support and improuvement of the OACt CCD cameras has continued as follows:
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.
|INAF Researchers:||V. Antonuccio, U. Becciani|
|Post Doc.:||A. Germaná|
|PhD. Students:||D. Ferro, A. Romeo|
|Technical Staff:||A. Costa, E. Martinetti|
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 particles per second were computed using 16 PEs on an IBM SP Power4 1300 MHz at Cineca.
FLY has a graphical Tcl/Tk interface that help the user to create all the
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 .
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.