Ref HAL: hal-00651233_v1
Ref Arxiv: 1112.0141
Ref. & Cit.: NASA ADS
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Commentaires: 14 pages, 6 figures, 6 tables, accepted for publication in Astronomy and Astrophysics

Ref HAL: hal-00651228_v1
Ref Arxiv: 1109.3619
DOI: 10.1051/0004-6361/201117463
Ref. & Cit.: NASA ADS
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58 citations

Ref Arxiv: 0
Résumé:

Gaia will provide a three-dimensional map of our Galaxy, with unprecedented positional and radial velocity measurements for about one per cent of the Galactic stellar population. Combined with astrophysical information derived from spectroscopy and photometry for each star, this will lead to a detailed understanding of Êthe formation, and dynamical and chemical evolution of our Galaxy. Other scientific products include extra-solar planets, minor bodies in the solar system, or distant quasars. The contribution of Gaia to stellar physics is less publicized, although very significant.Ê I show here a number of illustrative examples. For example, we will have access to very precise HR diagrams with very large sample of stars allowing extensive tests of fine effects in stellar evolution. Accurate parameters (esp. luminosities, and masses) will allow the independent determination of surface gravities, the characterization of non-LTE effects, and the derivation of more accurate chemical abundances. We will be able to quantify transport processes in various populations of stars, and shed new light on abundance anomalies. In addition, the preparation of Gaia induces a very large effort devoted to homogenize the stellar parameters of a great number of reference stars, and the development of performant tools designed to automatically extract parameters from tremendous amounts of spectra.

Ref Arxiv: 1011.2054
Ref. & Cit.: NASA ADS
Résumé:

During the evolution on the AGB, S-type stars are the first objects to experience s-process nucleosynthesis and third dredge-ups, and therefore to exhibit sprocess signatures in their atmospheres. Their significant mass loss rates (10^-7 to 10^-6 M*/year) make them major contributors to the AGB nucleosynthesis yields at solar metallicity. Precise abundance determinations in S stars are of the utmost importance for constraining e.g. the third dredge-up luminosity and efficiency (which has been only crudely parameterized in all current nucleosynthetic models so far). Here, dedicated S-star model atmospheres are used to determine precise abundances of key s-process elements, and to set constraints on nucleosynthesis and stellar evolution models. A special interest is paid to technetium, an element with no stable isotopes (99Tc, the only isotope produced by the s-process in AGB stars, has a half-life of 2.1 x 10^5 years). Its detection is considered as the best signature that the star effectively populates the thermally-pulsing AGB phase of evolution. The derived Tc/Zr abundances are compared, as a function of the derived [Zr/Fe] overabundances, with AGB stellar model predictions. The [Zr/Fe] overabundances are in good agreement with the model predictions, while the Tc/Zr abundances are slightly overpredicted. This discrepancy can help to set better constraints on nucleosynthesis and stellar evolution models of AGB stars.

Commentaires: 5 pages, 3 figures, To be published in the proceedings of the conference "Why Galaxies Care about AGB Stars II", held in Vienna, August 16-20, 2010; eds Franz Kerschbaum, Thomas Lebzelter, and Bob Wing, ASP Conf. Series

Ref Arxiv: 1011.2092
Ref. & Cit.: NASA ADS
Résumé:

S-type stars are late-type giants whose atmosphere is enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing AGB. A large grid of S-star model atmospheres has been computed covering the range 2700 < Teff < 4000 K with 0.5 < C/O < 0.99. ZrO and TiO band strength indices as well as VJHKL photometry are needed to disentangle Teff, C/O and [s/Fe]. A "best-model finding tool" was developed using a set of well-chosen indices and checked against photometry as well as low- and high-resolution spectroscopy. It is found that applying M-star model atmospheres (i.e., with a solar C/O ratio) to S stars can lead to errors on Teff up to 400K. We constrain the parameter space occupied by S stars of the vast sample of Henize stars in terms of Teff, [C/O] and [s/Fe].

Commentaires: 5 pages, 4 figures, "Why Galaxies Care About AGB Stars II" Conference, Vienna Aug. 2010, to appear in ASP Conference Series