Different Sodium enhancements among multiple populations of Milky Way globular clusters

TL;DR

This study investigates the variations in sodium abundances among different populations of Milky Way globular clusters, revealing links to their origins and formation histories.

Contribution

It provides a comprehensive analysis of Na abundance differences in 28 globular clusters, connecting these variations to their orbital properties and origins.

Findings

Na enrichment correlates with the Na abundance of first-generation stars.

Accreted clusters show larger Na enhancements than in situ clusters.

Na enrichment amplitude may reflect hierarchical galaxy formation processes.

Abstract

We searched for trails to understand the different Na abundances measured in first and second generation stars of ancient Milky Way globular clusters. For that purpose, we gathered from the recent literature the aforementioned Na abundances, orbital parameters, structural and internal dynamical properties and ages in an homogeneous scale of 28 globular clusters. We found that the intra-cluster Na enrichment, measured by the difference of Na abundances between first and second generation stars, exhibits a trend as a function of the Na abundances of first generation stars, in the sense that the more Na-poor the first generation stars, the larger the Na enrichment. By using the inclinations of the globular clusters' orbits, the analyzed Na enrichments also hinted at a boundary at ~0.3 dex to differentiate globular clusters with an accreted or in situ origin, the accreted globular clusters having larger Na enrichments. Because relatively larger intra-cluster Na enhancements are seen in accreted globular clusters, and small Na enhancements are observed in globular clusters formed in situ (although not exclusively), we speculate with the possibility that the amplitude of the Na enrichment could be linked with the building block paradigm. Globular clusters at the time of formation of first and second generation stars would seem to keep memory of this hierarchical galaxy formation process.