Spectroscopic studies of stellar populations in globular clusters and field stars: implications for globular cluster and Milky Way halo formation

TL;DR

This review discusses spectroscopic findings on multiple stellar populations in globular clusters, highlighting how initial mass, metallicity, and polluter types influence chemical diversity and formation processes in clusters and the Milky Way halo.

Contribution

It synthesizes spectroscopic evidence to elucidate the formation and chemical evolution of globular clusters and their role in the Milky Way halo.

Findings

Massive clusters host multiple populations with diverse chemistries.

Na/O and Mg/Al anti-correlations are more extended in metal-poor clusters.

Li abundance supports AGB stars as polluters for intermediate populations.

Abstract

We review spectroscopic results concerning multiple stellar populations in globularclusters. The cluster initial mass is the most important parameter determining the fraction of second generation stars. The threshold for the onset of the multiple population phenomenon is 1-3x10^5 MSun. Nucleosynthesis is influenced by metallicity: Na/O and Mg/Al anti-correlations are more extended in metal-poor than in metal-rich clusters. Massive clusters are more complex systems than the smaller ones, with several populations characterized by different chemical compositions. The high Li abundance observed in the intermediate second generation stars strongly favours intermediate mass AGB stars as polluters for this class of stars; however, it is well possible that the polluters of extreme second generation stars, that often do not have measurable Li, may be fast rotating massive stars or super-massive stars. The mass budget factor should be a function of the cluster mass, and needs to be large only in massive clusters.