The self-enrichment scenario in intermediate metallicity globular clusters

TL;DR

This study models intermediate mass asymptotic giant branch stars at low metallicity to evaluate their role in the self-enrichment of globular clusters, focusing on element abundance patterns.

Contribution

It provides updated stellar yields with explicit alpha-enhancement, testing the AGB self-enrichment scenario against observed globular cluster abundances.

Findings

Models agree with observed oxygen, aluminum, fluorine, and magnesium abundances.

Sodium predictions are uncertain due to nuclear reaction cross-section uncertainties.

Only stars with initial masses of 5-6 solar masses can significantly contribute to self-enrichment.

Abstract

We present stellar yields computed from detailed models of intermediate mass asymptotic giant branch stars of low metallicity. In this work, the whole main microphysics inputs have been updated, and in particular alpha-enhancement is explicitly taken into account both in the opacities and equation of state. The target of this work is to provide a basis to test the reliability of the AGB self-enrichment scenario for Globular Clusters of intermediate metallicity. These Globular Clusters exhibit well defined abundance patterns, which have often been interpreted as a consequence of the pollution of the interstellar medium by the ejecta of massive AGBs. We calculated a grid of intermediate mass models with metallicity Z=0.001; the evolutionary sequences are followed from the pre-Main sequence along the whole AGB phase. We focus our attention on those elements largely studied in the spectroscopic investigations of Globular Clusters stars, i.e. oxygen, sodium, aluminum, magnesium and fluorine.} The predictions of our models show an encouraging agreement with the demand of the self-enrichment scenario for what concerns the abundances of oxygen, aluminum, fluorine and magnesium. The question of sodium is more tricky, due to the large uncertainties of the cross-sections of the Ne-Na cycle. The present results show that only a relatively small range of initial masses (M=5,6 solar masses) can be responsible for the self enrichment.