Asymptotic Giant Branch models at very low metallicity

TL;DR

This paper models the evolution of a low-mass, very low metallicity star during the AGB phase, highlighting proton ingestion, deep dredge-up, and efficient s-process nucleosynthesis with detailed nuclear reaction networks.

Contribution

It presents the first detailed AGB evolution model at very low metallicity including proton ingestion and comprehensive nuclear network coupling.

Findings

High neutron density during proton ingestion activates the s-process.

Deep third dredge-up enriches the surface with 13C, 14N, and 7Li.

Detailed surface chemical evolution from proton ingestion to end of TP-AGB.

Abstract

In this paper we present the evolution of a low mass model (initial mass M=1.5 Msun) with a very low metal content (Z=5x10^{-5}, equivalent to [Fe/H]=-2.44). We find that, at the beginning of the AGB phase, protons are ingested from the envelope in the underlying convective shell generated by the first fully developed thermal pulse. This peculiar phase is followed by a deep third dredge up episode, which carries to the surface the freshly synthesized 13C, 14N and 7Li. A standard TP-AGB evolution, then, follows. During the proton ingestion phase, a very high neutron density is attained and the s-process is efficiently activated. We therefore adopt a nuclear network of about 700 isotopes, linked by more than 1200 reactions, and we couple it with the physical evolution of the model. We discuss in detail the evolution of the surface chemical composition, starting from the proton ingestion up to the end of the TP-AGB phase.