Hot bottom burning and s-process nucleosynthesis in massive AGB stars at the beginning of the thermally-pulsing phase

TL;DR

This study spectroscopically identifies massive Galactic AGB stars at the start of the thermal pulse phase, revealing high lithium abundance and weak s-process element enhancements, aligning with recent nucleosynthesis models.

Contribution

First spectroscopic identification of massive AGB stars at the beginning of the TP phase, providing new insights into HBB and s-process nucleosynthesis.

Findings

Massive AGBs are the most Li-rich observed to date.

Weak or no s-process element enhancements detected.

Tc element not observed, consistent with 22Ne neutron source dominance.

Abstract

We report the first spectroscopic identification of massive Galactic asymptotic giant branch (AGB) stars at the beginning of the thermal pulse (TP) phase. These stars are the most Li-rich massive AGBs found to date, super Li-rich AGBs with logE(Li)~3-4. The high Li overabundances are accompanied by weak or no s-process element (i.e. Rb and Zr) enhancements. A comparison of our observations with the most recent hot bottom burning (HBB) and s-process nucleosynthesis models confirms that HBB is strongly activated during the first TPs but the 22Ne neutron source needs many more TP and third dredge-up episodes to produce enough Rb at the stellar surface. We also show that the short-lived element Tc, usually used as an indicator of AGB genuineness, is not detected in massive AGBs which is in agreement with the theoretical predictions when the 22Ne neutron source dominates the s-process nucleosynthesis.