Self-enrichment in Globular Clusters: the extreme He-rich population of NGC 2808

TL;DR

This study investigates the chemical composition of the extreme helium-rich population in NGC 2808, proposing a new model involving AGB star yields and dust-driven mass loss to explain observed elemental abundances.

Contribution

It introduces a novel approach considering radiation pressure on dust in AGB star models to match observed chemical abundances in globular cluster stars.

Findings

AGB stars of 6.5-8 solar masses can produce the observed element abundances.

A new dust-driven mass loss model improves abundance predictions.

Deep mixing during RGB phase explains oxygen abundance spread.

Abstract

Almost several decades after the discovery of the first multiple populations in galactic globular clusters (GC) the debate on their formation is still extremely current and NGC2808 remains one of the best benchmark to test any scenario for their origin and the evolution. In this work we focus on the chemical composition of stars belonging to the extreme He-rich population populated by stars with the most extreme abundance of Mg, Al, Na, O and Si. We checked whether the most recent measures are consistent with the AGB yields of stars of $6.5-8~M_{\odot}$. These stars evolve on time scales of the order of 40-60 Myr and eject matter strongly enriched in helium, owing to a deep penetration of the surface convective zone down to regions touched by CNO nucleosynthesis occurring after the core He-burning phase. Since the big unknown of the AGB phase of massive stars is the mass loss, we propose a new approch that takes into account the effects of the radiation pressure on dust particles. We show that this more realistic description is able to reproduce the observed abundances of Mg, Al, Na and Si in these extreme stars. The large spread in the oxygen abundances is explained by invoking deep mixing during the RGB phase. It will be possible to check this work hypothesis as soon as the oxygen measurements of the main sequence stars of NGC2808 will be available.