Super AGB and AGB evolution and the chemical inventory in NGC 2419

TL;DR

This paper investigates the nucleosynthesis processes in massive AGB and Super AGB stars at low metallicity and their role in shaping the chemical composition of the globular cluster NGC 2419, highlighting model predictions and observational comparisons.

Contribution

It provides detailed models of HBB nucleosynthesis in low-metallicity AGB and SAGB stars and compares these yields with the chemical inventory of NGC 2419, offering insights into the cluster's formation.

Findings

Models predict Mg depletion and Si synthesis in massive AGB stars.

Predicted oxygen depletion in Mg-poor stars is very high.

High potassium abundance can be explained by proton captures on argon.

Abstract

We follow the scenario of formation of second generation stars in globular clusters by matter processed by hot bottom burning (HBB) in massive asymptotic giant branch (AGB) stars and Super AGB stars (SAGB). In the cluster NGC 2419 we assume the presence of an extreme population directly formed from the AGB and SAGB ejecta, so we can directly compare the yields for a metallicity Z=0.0003 with the chemical inventory of the cluster NGC 2419. At such a low metallicity, the HBB temperatures (well above 108K) allow a very advanced nucleosynthesis. Masses of about 6Mo deplete Mg and synthesize Si, going beyond Al, so this latter element results only moderately enhanced; sodium can not be enhanced. The models are consistent with the observations, although the predicted Mg depletion is not as strong as in the observed stars. We predict that the oxygen abundance must be depleted by a huge factor in the Mg poor stars. The HBB temperatures are close to the region where other p capture reactions on heavier nuclei become possible. We show that high potassium abundance found in Mg poor stars can be achieved during HBB, by p captures on the argon nuclei, if the relevant cross section(s) are larger than listed in the literature or if the HBB temperature is higher. Finally, we speculate that some calcium production is occurring owing to proton capture on potassium. We emphasize the importance of a strong effort to measure a larger sample of abundances in this cluster.