AGB subpopulations in the nearby globular cluster NGC 6397

TL;DR

This study analyzes the chemical compositions of AGB and RGB stars in NGC 6397, finding no deficit of Na-rich AGB stars, which supports current stellar evolution theories and contrasts with some other globular clusters.

Contribution

First detailed abundance analysis of AGB stars in NGC 6397, confirming the presence of Na-rich AGB stars and providing a control case for previous studies.

Findings

No deficit of Na-rich AGB stars in NGC 6397

AGB and RGB subpopulation ratios are consistent within uncertainties

Supports stellar theory predictions for AGB populations

Abstract

It has been well established that Galactic Globular clusters (GCs) harbour more than one stellar population, distinguishable by the anti-correlations of light element abundances (C-N, Na-O, and Mg-Al). These studies have been extended recently to the asymptotic giant branch (AGB). Here we investigate the AGB of NGC 6397 for the first time. We have performed an abundance analysis of high-resolution spectra of 47 RGB and 8 AGB stars, deriving Fe, Na, O, Mg and Al abundances. We find that NGC 6397 shows no evidence of a deficit in Na-rich AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB and RGB in NGC 6397 are identical, within uncertainties. This agrees with expectations from stellar theory. This GC acts as a control for our earlier work on the AGB of M 4 (with contrasting results), since the same tools and methods were used.