Evolution and Nucleosynthesis in Extremely Metal-Poor, Asymptotic Giant Branch Stars

Abstract

AbstractWe evolve extremely metal-poor ([Fe/H]≃–3), thermally pulsing Asymptotic Giant Branch (AGB) models with the mass range of 1–8 M⊙. The chemical yields ejected from the models are obtained by considering mass loss. We find that the 1- and 2-M⊙ AGB models are not affected by hot bottom burning (HBB). Nevertheless, they produce large amount of 7Li in an H-flash event. The occurrence of this event is associated with the ingestion of protons from the overlying H-rich envelope into the He convective shell driven by thermal pulse. The resulting 7Li abundances in the ejecta are higher than the primordial one predicted in Big-Bang nucleosynthesis. The efficient production of 7Li by the operation of HBB is also confirmed in the models of 4–8 M⊙. If these AGB stars have a low-mass companion, it is probable that mass loss from the primary AGB star brings the materials enriched in 7Li into the secondary star. This makes the surface composition of the secondary Li-rich. The formation of Li-rich stars, however, is strongly dependent on the mass loss history and binary separation. The nucleosynthesis for the other light nuclei is also calculated up to the end of the AGB phase. We find that the abundance patterns of the metal-poor stars CS 29528–041 and CS 29497–030 are well reproduced by yields from our AGB models.