A deep insight into the Mg-Al nucleosynthesis in massive AGBs and SAGB stars

TL;DR

This paper investigates magnesium, aluminium, and silicon nucleosynthesis in massive AGB and SAGB stars, highlighting the impact of proton capture rates on observed abundance patterns in globular clusters.

Contribution

It introduces a revised proton capture rate on Mg25 that better explains observed Mg-Al and Al-Si abundance correlations in globular cluster stars.

Findings

Enhanced Mg25 proton capture rate reproduces Mg depletion in models.

Models show positive Al-Si correlation consistent with observations.

Discussion of uncertainties in nuclear physics inputs.

Abstract

The stars in globular clusters are known to differ in their surface chemistry: the spectroscopic investigations in the last decades outlined the presence of star-to-star differences in the abundances of the light elements, up to aluminium (and possibly silicon), suggesting that some stars were contaminated by an advanced proton-capture nucleosynthesis. The AGB stars are one of the most promising candidates in producing the pollution of the intra-cluster medium, via the ejection of gas processed by Hot Bottom Burning, from which new stellar generations are formed. This work is focused on the degree of nucleosynthesis involving magnesium, aluminium and silicon that these sources may experience. The key ingredient to determine the degree of magnesium depletion, and the amount of aluminium that can be produced, is the rate of proton capture on Mg25, forming Al26; an increase in this cross-section by a factor 2 with respect to the highest value allowed by the NACRE compilation allows to reproduce the extent of the Mg-depletion observed, and is in qualitative agreement with the positive Al-Si correlation observed in a few clusters. The main uncertainties associated with the macro- and micro-physics input, are discussed and commented, and the comparison with recent spectroscopic results for the globular cluster showing some degree of Mg-Al anticorrelation and Al-Si correlation is presented.