CEMP-s Stars: AGB Yield Predictions and Thermohaline Mixing

Details CEMP-s Stars: AGB Yield Predictions and Thermohaline Mixing CEMP-s Stars: AGB Yield Predictions and Thermohaline Mixing

Mar 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aipc..990..330b&link_type=abstract

FIRST STARS III: First Stars II Conference. AIP Conference Proceedings, Volume 990, pp. 330-332 (2008).

Computer Science

4

Population Ii Stars, Abundances, Chemical Composition, Stellar Structure, Interiors, Evolution, Nucleosynthesis, Ages, Mass Loss And Stellar Winds, Binary And Multiple Stars, Carbon Stars, S Stars, And Related Types

Scientific paper

CS 29497-030 and CS 31062-050 belong to a sample of C-rich, s-process rich and extremely metal-poor stars (CEMP-s+r). To explain the s-process enrichment, we considered these stars to be extrinsic asymptotic giant branch (AGB) stars, belonging to binary systems where the more massive AGB companion polluted the observed star (of ~0.8 Msolar) with efficient stellar winds. To explain the r-process enrichment, we assumed that the parental cloud was already enriched in r-process elements.
For the main sequence CS 29497-030 we hypothesize that the primary AGB had an initial mass of ~1.3 Msolar and underwent a very limited number of third dredge up episodes. A very small dilution between AGB winds and envelope mass of the observed star is derived by comparing AGB nucleosynthesis yields and observed abundances, consistent with the fact that dwarf stars of ~0.8 Msolar are characterized by a limited subphotospheric convective zone. This is compatible with moderate thermohaline mixing (e.g., [l]). AGB models of higher initial mass undergo an increasing number of third dredge up (TDU) episodes and produce larger carbon and s-process abundances at the surface. For AGB models of 1.5 Msolar and 2 Msolar a good match with the observed s-process abundance distribution can still be found, provided a dilution factor of 0.5 dex or 0.8 dex is applied. The predicted yields of Na and Mg, which are extremely sensitive to the number of thermal pulses, however, would be much higher than observed.
CS 31062-050 is a red subgiant that has likely undergone the first dredge up episode, where the convective envelope extends over about 80% of the stellar mass, erasing any effect of thermohaline mixing. The ~1.3 Msolar AGB model will fit the observed elemental distribution, but will only be compatible with a quite large amount of mass accreted by the AGB donor. For this star, AGB models of 1.5 Msolar to 2 Msolar and dilution factors of 1.0 dex to 1.3 dex may be more appropriate, including the reproduction of [Mg/Fe].

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