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First Stars III
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Poster
Title: Contributions from the First AGB Stars: Predicted Stellar Yields of Low-Metallicity Carbon-Enhanced AGB Stars, & Thermohaline Mixing Author(s): Inese Ivans Abstract: A complete picture of Galactic Chemical Evolution requires an understanding of the production of neutron-capture elements. Approximately half of these are produced in slow-neutron-capture process (s-process) during the recurrent thermal pulse phase of low to intermediate mass stars (AGB). All but the present generation of such stars are dead. However, roughly 1/5 of them had companions of lower mass that can still be observed today, upon whose photospheres AGB stellar winds deposited chemically-enriched material. Two such stars, CS29497-030 and CS31062-050, belong to a large sample of C-rich, s-process rich and extremely metal-poor stars (CEMP-s). We have calculated nucleosynthetic yields in order to match the observed abundances of both stars. Our hypothesis is that the primary AGB had an initial mass of approximately 1.3 MSun that underwent a very limited number of third dredge up episodes. (AGB models of initial mass 1.5 to 3 MSun undergo an increasing number of third dredge up episodes, producing larger carbon and s-process abundances at the surface). No dilution between the AGB winds and the envelope of the observed star was required, consistent with the fact that stars of approximately 0.8 MSun are characterized by a limited subphotospheric convective zone, and the dilution factor is negligible. However, the mean molecular weight of the transferred mass is higher than that of the original composition of the binary system. Consequently, efficient thermohaline mixing (TM) is to be expected. Incorporating first estimates of TM in these stars, a dilution on the order of one dex and a range of AGB initial masses 1.5 to 2 MSUN appear to fit the observations equally well. Two elements, Na and Mg, which are efficiently produced in the thermal pulses by neutron captures may provide strong constraints on the efficiency of the TM, whose treatment cannot be precisely constrained in the framework of present knowledge.
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