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

As a part of a systematic study of magnetic activity manifestations in late-type single stars and close binary systems, S. Messina has carried out during the course of 2004, in collaboration with M. Zboril of Astronomical Institute of Slovak Academy of Science, a search for activity-induced variability in the SB2 systems AR Lac, KT Peg and KZ And as well as in the SB1 systems II Peg and EI Eri in the September 1997 season (Zboril et al. ()). The study of H$\alpha$ line profiles, of Na I 5889Å  doublet and Ca II IRT lines and TiO 7055Å  absorption band is presented. The spot properties were estimated for the systems AR Lac and II Peg. The former possesses spots at intermediate stellar latitudes on the primary component while the latter shows these photospheric inhomogeneities at intermediate up to high latitudes. In both cases the spots cooler than surrounding photosphere were revealed. Significant variability in hydrogen H$\alpha$ line profile was recognised on the systems II Peg and EI Eri and evidence of rotational modulation on the systems KT Peg and II Peg.

In collaboration with the Gomez, M. T., Terranegra, L. and Andretta, V. from the Naples stellar group, I. Busá is involved in a large project on the study of activity in stars of spectral type similar, or later than the Sun, from PMS to ZAMS, based on the analysis of high resolution CaII InfraRed Triplet (CaIIIRT: $\lambda=$ 8498, 8542, 8662 Å) observed profiles. In the framework of stellar chromospheric activity several authors have infact underscored the diagnostic power of these lines as activity indicators. Nevertheless the proper subtraction of the photospheric contribution, essential for the correct estimation of the chromospheric contribution to any activity diagnostic, has never been taken into account. I. Busá has worked on the definition and calibration of a purely chromospheric index \ensuremath{R _\mathrm{IRT}} which promise to be a good probe of stellar activity, at least in main-sequence stars (()).

Figure: The \ensuremath{R _\mathrm{IRT}} index (calculated for the $\lambda$8542 CaIIIRT line as $CD_{calculated}$ - $CD_{observed}$) versus \ensuremath{\log{R^{'}_\mathrm{HK}}}for a sample of stars observed with the SARG spectrograph at TNG .

Fig. 1.9 shows the \ensuremath{R _\mathrm{IRT}} index of the $\lambda$8542 CaIIIRT line versus \ensuremath{\log{R^{'}_\mathrm{HK}}} for a set of 40 stars covering the spectral types from F5 to K3. and spanning a wide range of activity level. The observations were collected by I. Busá with the SARG, the high-dispersion spectrograph at the 3.5 m TNG in La Palma (Spain) on February 2002.

The study of the average photospheric temperature variation based on line-depth ratios (LDR) has been continued by Catalano, Frasca, Marilli and Biazzo. They have shown as this method permits to measure the modulation of the average surface temperature with sensitivity around 10 Kelvin degrees in two classes of variable stars, i.e. spotted active stars and Cepheid pulsating variables (,). In particular, they have applied a spot model to simultaneous light and temperature curves of VY Ari, IM Peg and HK Lac, for an unambiguous determination of spot temperatures ($T_{\rm sp}$) and areas ($A_{\rm
rel}$) (see Fig. 1.10 for an example).

Figure 1.10: Observed (dots) and synthetic (full lines) light and temperature curves of VY Ari. The $B-V$ curve is also shown in the lower box with the solution superimposed for comparison. A schematic map of the starspot distribution, as seen at two different rotational phases, is also shown.

Grids of solutions with comparable $\chi$-square have been found for a wide range of spot temperatures. However, the behavior of the solution grids for temperature and light curves in the $T_{\rm sp}$-$A_{\rm
rel}$ plane is very different and a rather small and unique intersection area can be found (Fig. 1.11).

Figure 1.11: Grids of solutions for VY Ari. The filled circles represent the solutions for light curve, while the filled diamonds represent the solutions for temperature curve. The hatched area, in each box, is the locus of the allowed solutions accounting for data errors.

They have searched for simultaneous solutions of the temperature and light curves by using the spectral energy distribution (SED) predicted by model atmospheres (Kurucz and NextGen) to evaluate the continuum flux ratio between the spots and the quiet photosphere. Spot temperatures deduced under the black-body approximation for the flux ratio have been also computed in order to compare their findings with previous results on the same stars. The spot temperatures derived with the black-body SED are lower than those obtained with the model atmosphere SED. The results from Kurucz and NextGen models are instead in good agreement.

The temperature difference between spots and photosphere, a parameter tied to the blocking of the convection produced by starspot magnetic fields, is in the range 450-850K for RS CVn stars. These values are significantly smaller than those typical of sunspot umbrae, and are probably more representative of a weighted average of umbral and penumbral temperatures ().

They have also studied several young solar-type stars with activity levels intermediate between the Sun and the very active RS CVn binaries. For this project, a collaboration with G.W. Henry (Tennessee State University) was started. Contemporaneous spectroscopic data obtained at Catania Observatory (Serra La Nave station, Mt. Etna) and photometric data acquired in the Strömgren bands with the T4 0.75-m Automatic Photometric Telescope (APT) at Fairborn Observatory (Arizona, USA) have been analysed(,). Surface inhomogeneities have been detected from the rotational modulation of stellar brightness as well as from the modulation of several photospheric line-depth ratios (LDRs). The presence of chromospheric plage-like regions has been inferred from the rotational modulation of the H$\alpha$ line equivalent width ( $EW_{\rm H\alpha}$) evaluated with the spectral synthesis method. The most relevant results are the strong correlation between the brightness and temperature curves derived respectively from photometry and the LDRs as well as the striking anti-correlation between brightness and H$\alpha$ emission. This suggests a close spatial association of spots and plages, as frequently observed for the largest sunspot groups and for some very active RS CVn systems. Moreover, a simple spot/plage model applied to the observed flux curves allows a rough reconstruction of photospheric and chromospheric features of young main sequence stars ().

Moreover, from the combined analysis of contemporaneous temperature and light curves the authors have obtained unique solutions of the spot temperature and the spot coverage factor for the solar-type star k1 Cet by using the same method developed by Frasca et al. (). Also for $\epsilon$ Eri and HD 166 they could evaluate the spot parameters. They derived spot temperatures closer to solar spot penumbrae rather than umbrae. The spot relative coverages of these stars are much smaller than in the spotted RS CVn stars and are closer to the largest solar spot groups.

Spectroscopic and photometric observations of the young F5-6V field star HD 111456 have been performed to characterize this very active star (Freire Ferrero et al. ()). Although the limit of magnetic activity detectability at optical wavelengths occurs around F5-6 spectral types, HD 111456 shows unusual chromospheric and TR activity, as confirmed by the high UV fluxes in the CII, CIV, SiII and SiIV lines and by our detection of core emission in the CaII H and K lines as well as by excess H$\alpha$ and H$\beta$ emission and HeI D$_3$ absorption. The CaII and H$\alpha$ chromospheric emission fluxes do not show any detectable variation on time scales of few days, as well as $uvby$ photometry. A possible explanation in terms of a very high level of magnetic activity and a homogeneous distribution of active regions, both at chromospheric and photospheric level, or a very low inclination of the rotation axis with respect to the line of sight is proposed. From 1998 to 2001 the radial velocity shows clear variations indicating a possible binary system, not confirmed by the spectral energy distribution.

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Next: Systematic observations and activity Up: Magnetic activity and variability Previous: Magnetic activity and dynamics   Contents   Index
Innocenza Busa' 2005-11-14