Production of Lithium and Heavy Elements in AGB Stars Experiencing PIEs

TL;DR

This study investigates lithium and heavy element production in AGB stars during proton ingestion events, revealing significant nucleosynthesis of these elements, especially in low-metallicity stars, and suggests a link to J-type carbon stars.

Contribution

It provides the first detailed modeling of lithium and heavy element production during PIEs in AGB stars across a range of masses and metallicities, with publicly available yields.

Findings

Lithium is produced with surface abundances A(Li) 3-5.

Surface enrichment of lithium increases with decreasing stellar mass.

Heavy elements like Sr, Ba, Eu, Pb are significantly produced at low metallicities.

Abstract

Asymptotic giant branch (AGB) stars can experience proton ingestion events (PIEs), leading to a rich nucleosynthesis. During a PIE, the intermediate neutron capture process (i-process) develops, leading to the production of trans-iron elements. It is also suggested that lithium is produced during these events. We investigate the production of lithium and trans-iron elements in AGB stars experiencing a PIE with $1<M_{\rm ini}/M_{\odot}< 3$ and $-3< \mathrm{[Fe/H]} <0$. We find that lithium is produced in all PIE models with surface abundances $3<$ A(Li) $<5$. The surface enrichment and overall AGB lithium yield increases with decreasing stellar mass. The lithium enrichment is accompanied by a production of $^{13}$C with $3<^{12}$C/$^{13}$C$<9$ at the surface just after the PIE. AGB stars experiencing PIE may be related to J-type carbon stars whose main features are excesses of lithium and $^{13}$C. In addition to Li and $^{13}$C, heavy elements (e.g., Sr, Ba, Eu, Pb) are significantly produced in low-metallicity stars up to [Fe/H]$\simeq-1$. The yields of our models are publicly available. Additionally, of interest to the Li nucleosynthesis, we provide an updated fitting formula for the $^{7}$Be($e^-,\nu_e$)$^{7}$Li electron capture rate.