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First Stars III
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Talk
Title: Carbon-enhanced metal-poor stars: effects of binary evolution at low metallicity Presenter: Onno Pols Abstract: A large fraction, perhaps 25%, of extremely metal-poor stars in the Galactic halo are carbon-rich objects, with enhancements of carbon by as much as a factor 100. These carbon-enhanced metal-poor (CEMP) stars exhibit a variety of abundance patterns, but the majority (the CEMP-s stars) show enhancements of heavy s-process elements, in particular of lead. Many of these stars have been found to be spectroscopic binary systems. Their binarity and abundance patterns strongly suggest that at least the CEMP-s stars have been polluted by a companion star while in an advanced stage of evolution, which has long since faded away. We will present the first results of our efforts to understand the nature of CEMP stars by means of a combination of detailed binary evolution and nucleosynthesis models. Using a population synthesis technique we investigate whether the high frequency and the abundance patterns of CEMP-s stars can be explained in the binary scenario. An often-made assumption is that the material accreted from the companion remains on the surface of the star until first dredge-up occurs on the red giant branch. We show that the process of thermohaline convection changes this picture drastically by rapidly mixing the accrested material down to the central parts of the star, with dramatic effects on the surface abundances.
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