Flash‐Driven Convective Mixing in Low‐Mass, Metal‐deficient Asymptotic Giant Branch Stars: A New Paradigm for Lithium Enrichment and a Possible s‐Process

Abstract

We have calculated models for low-mass, metal-deficient ([Fe/H] = -2.7) stars from the zero-age main sequence through the thermally pulsing asymptotic giant branch (TP-AGB) phase. We confirm that the entropy barrier between the H-rich envelope and the He intershell can be surmounted by the energy released by thermal pulses during the early phase of the TP-AGB. For models in the mass range of 1 ≤ M / M ☉ < 3, this energy release causes the top of the flash-driven convective shell to reach into the bottom of the overlying H-rich envelope. Protons are then carried downward into the hotter He- and 12 C-rich layer, while He intershell material is mixed upward. This phenomenon causes the surface chemical composition to change dramatically. In particular, surface abundances are enriched in CNO elements by as much as 1 to 3 orders of magnitude. Lithium is also enhanced by this event in the 1, 1.5, and 2 M ☉ models. We have also studied the formation and reactions of 13 C as protons are mixed into the He intershell. We find that this mixed material experiences the s -process through the α-capture reaction on newly synthesized 13 C under convective conditions during the thermal pulse. This results in neutron-capture nucleosynthesis under relatively high neutron density environments. In lower mass models, the s -abundance distributions would be characterized by the small number of neutron irradiations through the standard s -process, which occurs under radiative conditions in a 13 C pocket as a result of the immediate termination of the third dredge-up. Accordingly, in extremely metal-poor stars, we may observe the s -element distributions mainly created by the s -processing relevant to the proton-mixing event. Furthermore, we discuss possible observational signatures of the mixing of protons into He-burning regions.