NGC 6535: the lowest mass Milky Way globular cluster with a Na-O anti-correlation? Cluster mass and age in the multiple population context

TL;DR

This study analyzes the chemical composition of NGC 6535, a low-mass Milky Way globular cluster, revealing it exhibits the Na-O anti-correlation typical of multiple populations, and discusses how cluster mass and age influence this phenomenon.

Contribution

It provides the first detailed chemical analysis of NGC 6535, confirming the presence of multiple populations in the lowest mass cluster known with such features.

Findings

NGC 6535 shows a clear Na-O anti-correlation.

The cluster has a metallicity of [Fe/H]=-1.95.

Mass and age are key factors in the presence of multiple populations.

Abstract

To understand globular clusters (GCs) we need to comprehend how their formation process was able to produce their abundance distribution of light elements. In particular, we seek to figure out which stars imprinted the peculiar chemical signature of GCs. One of the best ways is to study the light-element anti-correlations in a large sample of GCs that are analysed homogeneously. As part of our spectroscopic survey of GCs with FLAMES, we present here the results of our study of about 30 red giant member stars in the low-mass, low-metallicity Milky Way cluster NGC 6535. We measured the metallicity (finding [Fe/H]=-1.95, rms=0.04 dex in our homogeneous scale) and other elements of the cluster and, in particular, we concentrate here on O and Na abundances. These elements define the normal Na-O anti-correlation of classical GCs, making NGC 6535 perhaps the lowest mass cluster with a confirmed presence of multiple populations. We updated the census of Galactic and extragalactic GCs for which a statement on the presence or absence of multiple populations can be made on the basis of high-resolution spectroscopy preferentially, or photometry and low-resolution spectroscopy otherwise; we also discuss the importance of mass and age of the clusters as factors for multiple populations.