Different sodium enhancements among multiple populations of Milky Way globular clusters

Abstract

We searched for clues to understand the different Na abundances measured in first and second generation stars of ancient Milky Way globular clusters. For that purpose, from the recent literature, we gathered the aforementioned Na abundances, orbital parameters, and structural and internal dynamical properties and ages in a homogeneous scale of 28 globular clusters. We found that the intra-cluster Na enrichment, which is 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 are, the larger the Na enrichment is. 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 that the amplitude of the Na enrichment may be linked with the building block paradigm. Globular clusters at the time of formation of first and second generation stars would seem to keep a memory of this hierarchical galaxy formation process.