Lithium abundances as a tracer of AGB stars pollution in the globular cluster NGC 6752

TL;DR

This study analyzes lithium and aluminium abundances in 217 stars of globular cluster NGC 6752 to understand pollution sources and reveals evidence of lithium production in AGB stars, challenging previous assumptions.

Contribution

It provides new evidence of lithium production in AGB stars within a globular cluster, highlighting complex chemical enrichment processes.

Findings

Identified a lithium plateau at A(Li)=2.33 dex in unevolved stars.

Discovered Al-rich stars with unexpectedly high lithium content.

Observed a continuous decrease of lithium from the turn-off to red giant branch.

Abstract

This paper presents the chemical abundance analysis of 217 stars in the metal-poor globular cluster NGC 6752, distributed from the turn-off to the lower red giant branch. Aluminium and Lithium abundances were derived through spectral synthesis applied to spectra collected with FLAMES, both in GIRAFFE and UVES modes. The work aims to gain insight into the nature of the polluter(s) responsible for the abundance variations and C-N, Na-O, Al-Mg anti-correlations associated with the multiple population phenomenon. We found a plateau at A(Li)=2.33$\pm$0.06 dex in unevolved stars, with the average Li content decreasing continuously down to $\sim$1.25 dex at the bottom of the red giant branch. As expected in the classic anti-correlation scenario, we found stars low in Al and high Li abundance and stars high in Al and low in Li. However, in addition, we also found evidence of Al-rich, second-generation stars with high Li content. This finding suggests the need for Li production, known to happen in intermediate-mass ($\sim$4-8M$_{sun}$) AGB stars through the Cameron-Fowler mechanism. It is worth noticing that the Li abundance observed in Al-rich stars never exceeds that in Al-poor stars.