Tracing Stellar Populations through the Mg-Al Anti-Correlation in Gaia-ESO Globular Clusters

Abstract

Context. Globular clusters (GCs) are fragments of the chemical history and evolution of the Milky Way. Their notable intra-cluster abundance variations have been used to characterise chemical trends and stellar populations. However, the origins of these populations and GCs in general are still largely unknown. Aims. Our aim is to explore the origin of the stellar populations belonging to 14 GCs making use of their abundances derived from the Gaia-ESO Survey (GES) spectra. In particular, we use the Mg-Al anti-correlation to separate the populations. Methods. The GES spectra were revisited to derive additional homogenous chemical abundances for Mg and Al. A Gaussian Mixture Model (GMM) was applied to the [Mg/Al] distribution for each cluster to define a threshold between the two main populations. Results. Eight clusters imply the presence of Mg-Al anti-correlations and dual populations. We find there is a regime transition near [Fe/H] approximately -1.4, where, the Mg-Al anti-correlation disappears at higher metallicity. Several clusters, regardless of their Mg-Al anti- correlation behaviour, exhibit metallicity spreads (e.g., NGC 2808, NGC 6752) and s-process variations (e.g., NGC 7078, NGC 1851), suggesting complex enrichment histories. Conclusions. The presence and spread of the Mg-Al anti-correlation depend on cluster mass and metallicity, with metal poor and/or massive clusters exhibiting the clearest anti-correlation. This work demonstrates additional abundances that can be retrieved from Gaia-ESO Survey spectra and provides further insight into the chemistry of multiple stellar populations within GCs. These findings highlight the chemical complexity of GCs and the need for large, homogeneous datasets to trace their formation histories.