Revisiting nucleosynthesis in globular clusters: the case of NGC 2808 and the role of He and K

TL;DR

This study investigates the nucleosynthesis processes in globular cluster NGC 2808, focusing on the roles of helium and potassium, to understand the chemical enrichment patterns and their relation to stellar populations.

Contribution

It provides detailed nucleosynthesis calculations across a range of temperatures, highlighting the constraints on polluter sources and the conditions producing observed element correlations.

Findings

Mg is a key indicator of H-burning temperature, suggesting 70-80 MK for NGC 2808.

Potassium can be produced at observed levels at temperatures above 180 MK.

Current polluter models do not fully explain the observed chemical abundance patterns.

Abstract

Motivated by recent reports concerning the observation of limited enrichment in He but excess K in stars of globular clusters, we revisit the H-burning conditions that lead to the chemical properties of multiple stellar populations in these systems. In particular, we are interested in correlations of He and K with other elements, like O, Na, Al, Mg and Si, reported in stars of NGC 2808. We perform calculations of nucleosynthesis at constant temperature and density, exploring the temperature range of 25 to 200 10$^6$ K (25 to 200 MK), using a detailed nuclear reaction network and the most up-to-date nuclear reaction rates. We find that Mg is the most sensitive "thermometer" of hydrostatic H-burning conditions, pointing to a temperature range of 70-80 MK for NGC~2808, while He is a lesser - but not negligible - constraint. K can be produced at the levels reported for NGC 2808 at temperatures $>$180 MK and Si at T$>$80 MK. However, in the former temperature range Al and Na are totally destroyed and no correlation can be obtained, in contrast to the reported observations. None of the putative polluter sources proposed so far seem to satisfy the ensemble of nucleosynthesis constraints.