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See detailA Comparison of Methods for Photospheric Abundance Determinations in K-Type Stars
Affer, L.; Micela, G.; Morel, Thierry ULg et al

in Chemical Abundances and Mixing in Stars in the Milky Way and its Satellites, ESO ASTROPHYSICS SYMPOSIA. ISBN 978-3-540-34135-2. Springer-Verlag, 2006, p. 31 (2006)

We have performed a detailed abundance analysis of six inactive K-type stars using high-resolution optical spectra. We have used three different techniques and compared the results obtained in order to ... [more ▼]

We have performed a detailed abundance analysis of six inactive K-type stars using high-resolution optical spectra. We have used three different techniques and compared the results obtained in order to establish their respective merits and faults. The two spectroscopic methods give consistent results suggesting that non- LTE effects are small, whereas the â mixedâ spectroscopic-photometric method leads to photospheric parameters and abundances systematically lower than those obtained with the other two. We have also determined the starsâ positions in H-R diagrams and made a comparison between the gravities derived from the ionization equilibrium of the iron lines and from the evolutionary tracks: the agreement is reasonably good. [less ▲]

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See detailSome Concerns About the Reliability of LTE Abundance Analyses in Cool, Active Stars
Morel, Thierry ULg; Micela, G.; Favata, F.

in Chemical Abundances and Mixing in Stars in the Milky Way and its Satellites, ESO ASTROPHYSICS SYMPOSIA. ISBN 978-3-540-34135-2. Springer-Verlag, 2006, p. 64 (2006)

We discuss recent observational evidence illustrating the current limitations plaguing classical LTE abundance analyses of cool (T [SUB]eff[/SUB] < 5500 K), chromospherically active stars. Although ... [more ▼]

We discuss recent observational evidence illustrating the current limitations plaguing classical LTE abundance analyses of cool (T [SUB]eff[/SUB] < 5500 K), chromospherically active stars. Although significant progress on this issue can be evidently expected from a more realistic atmospheric modelling and treatment of NLTE line formation, a homogeneous abundance study of a large sample of inactive K-type stars may also prove valuable in disentangling temperature and activity effects. [less ▲]

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See detailThe life of stars and their planets
Catala, C.; Aerts, C.; Aigrain, S. et al

in Favata, F.; Sanz-Forcada, J.; Giménez, A. (Eds.) et al 39TH ESLAB Symposium on Trends in Space Science and Cosmic Vision 2020 (2005, December 01)

We lack a reliable scenario for the formation and evolution of stars and their planetary systems, involving key factors such as magnetic fields and turbulence. We present the case for a mission concept ... [more ▼]

We lack a reliable scenario for the formation and evolution of stars and their planetary systems, involving key factors such as magnetic fields and turbulence. We present the case for a mission concept that will clarify these problems and give us a global view of the evolution of combined star and planetary systems. This will be achieved by simultaneously addressing the search for planetary transits in front of a large number of stars, including many nearby stars, the study of their internal structure and evolution via asteroseismology, and that of their magnetic activity, via UV monitoring. [less ▲]

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See detailSpectroscopic determination of photospheric parameters and chemical abundances of 6 K-type stars
Affer, L.; Micela, G.; Morel, Thierry ULg et al

in Astronomy and Astrophysics (2005), 433

High resolution, high -S/N- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed ... [more ▼]

High resolution, high -S/N- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed photospheric parameters and abundances and to compare the characteristics of analysis techniques. The results have been compared with the aim of determining the most robust method to perform complete spectroscopic analyses of K-type stars, and in this perspective the present work must be considered as a pilot study. In this context we have determined the abundance ratios with respect to iron of several elements. In the first method the photospheric parameters (T_eff, log g, and ξ) and metal abundances are derived using measured equivalent widths and Kurucz LTE model atmospheres as input for the MOOG software code. The analysis proceeds in an iterative way, and relies on the excitation equilibrium of the ion{Fe}{i} lines for determining the effective temperature and microturbulence, and on the ionization equilibrium of the ion{Fe}{i} and ion{Fe}{ii} lines for determining the surface gravity and the metallicity. The second method follows a similar approach, but discards the ion{Fe}{i} low excitation potential transitions (which are potentially affected by non-LTE effects) from the initial line list, and relies on the B-V colour index to determine the temperature. The third method relies on the detailed fitting of the 6162 à ion{Ca}{i} line to derive the surface gravity, using the same restricted line list as the second method. Methods 1 and 3 give consistent results for the program stars; in particular the comparison between the results obtained shows that the ion{Fe}{i} low-excitation potential transitions do not appear significantly affected by non-LTE effects (at least for the subgiant stars), as suggested by the good agreement of the atmospheric parameters and chemical abundances derived. The second method leads to systematically lower T_eff and log g values with respect to the first one, and a similar trend is shown by the chemical abundances (with the exception of the oxygen abundance). These differences, apart from residual non-LTE effects, may be a consequence of the colour-T_eff scale used. The α-elements have abundance ratios consistent with the solar values for all the program stars, as expected for â normalâ disk stars. The first method appears to be the most reliable one, as it is self-consistent, it always leads to convergent solutions and the results obtained are in good agreement with previous determinations in the literature. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roche de los Muchachos of the Instituto de Astrofisica de Canarias. Table 6 is only available in electronic form at http://www.edpsciences.org [less ▲]

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See detailPhotospheric abundance peculiarities in chromospherically active stars
Morel, Thierry ULg; Micela, G.; Favata, F.

in 13th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun (2005, March 01)

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See detailThe photospheric abundances of active binaries. III. Abundance peculiarities at high activity levels
Morel, Thierry ULg; Micela, G.; Favata, F. et al

in Astronomy and Astrophysics (2004), 426

We report the determination from high-resolution spectra of the atmospheric parameters and abundances of 13 chemical species (among which lithium) in 8 single-lined active binaries. These data are ... [more ▼]

We report the determination from high-resolution spectra of the atmospheric parameters and abundances of 13 chemical species (among which lithium) in 8 single-lined active binaries. These data are combined with our previous results for 6 other RS CVn systems to examine a possible relationship between the photospheric abundance patterns and the stellar activity level. The stars analyzed are generally found to exhibit peculiar abundance ratios compared to inactive, Galactic disk stars of similar metallicities. We argue that this behaviour is unlikely to be an artefact of errors in the determination of the atmospheric parameters or non-standard mixing processes along the red giant branch, but diagnoses instead the combined action of various physical processes related to activity. The most promising candidates are cool spot groups covering a very substantial fraction of the stellar photosphere or NLTE effects arising from nonthermal excitation. However, we cannot exclude the possibility that more general shortcomings in our understanding of K-type stars (e.g. inadequacies in the atmospheric models) also play a significant role. Lastly, we call attention to the unreliability of the (V-R) and (V-I) colour indices as temperature indicators in chromospherically active stars. Based on observations collected at the European Southern Observatory, Chile (Proposals 64.L-0249 and 071.D-0260). Table A.1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/426/1007 [less ▲]

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See detailThe Photospheric Abundances of Active Binaries: First Results
Morel, Thierry ULg; Micela, G.; Favata, F. et al

in Stars as Suns : Activity, Evolution and Planets (2004, January 01)

The coronal abundances of chromospherically active binaries have come under close scrutiny in recent years thanks to several X-ray satellites such as XMM or Chandra. Unfortunately a detailed analysis of ... [more ▼]

The coronal abundances of chromospherically active binaries have come under close scrutiny in recent years thanks to several X-ray satellites such as XMM or Chandra. Unfortunately a detailed analysis of the chemical fractionation processes that are possibly operating between the photosphere and the corona is plagued by the lack of reliable and accurate photospheric abundance determinations. In particular a detailed investigation of the photospheric metal content seems worthwhile in the light of several studies that have questioned the customary use of photometric indices as effective temperature indicators in active binaries. We shall present our ongoing effort to determine in a self-consistent way accurate physical parameters and photospheric abundances for a large number of systems and discuss the implications of our results on their abundance patterns and evolutionary status. Preliminary results suggest that these systems are not as iron-deficient as found in previous studies and indicate an overabundance of several key elements (e.g. Na Mg Al Ca) with respect to the solar pattern. [less ▲]

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See detailThe photospheric abundances of active binaries. II. Atmospheric parameters and abundance patterns for 6 single-lined RS CVn systems
Morel, Thierry ULg; Micela, G.; Favata, F. et al

in Astronomy and Astrophysics (2003), 412

Photospheric parameters and abundances are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Abundances have been ... [more ▼]

Photospheric parameters and abundances are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Abundances have been derived for 13 chemical species, including several key elements such as Li, Mg, and Ca. Two methods have been used. The effective temperatures, surface gravities and microturbulent velocities were first derived from a fully self-consistent analysis of the spectra, whereby the temperature is determined from the excitation equilibrium of the Fe I lines. The second approach relies on temperatures derived from the (B-V) colour index. These two methods give broadly consistent results for the stars in our sample, suggesting that the neutral iron lines are formed under conditions close to LTE. We discuss the reliability in the context of chromospherically active stars of various colour indices used as temperature indicators, and conclude that the (V-R) and (V-I) colours are likely to be significantly affected by activity processes. Irrespective of the method used, our results indicate that the X-ray active binaries studied are not as metal poor as previously claimed, but are at most mildly iron-depleted relative to the Sun (-0.41protect <~ [Fe/H]protect la +0.11). A significant overabundance of several chemical species is observed (e.g., the alpha -synthezised elements). These abundance patterns are discussed in relation to stellar activity. Based on observations collected at ESO (La Silla, Chile). Table A.1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/495 [less ▲]

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