Constraining the Origin of Multiple Stellar Populations in Stellar Clusters
Globular clusters were among the first luminous objects to form in the Universe. They are dense collections of hundreds of thousands of stars. Globular cluster formation is a major unsolved problem in astrophysics. A new constraint on the problem came from the discovery of unexpected star-to-star variations in the abundances of some light elements. These abundance variations (or multiple stellar populations) are ubiquitous to all globular clusters studied to date. The pursuit to explain this longstanding prob- lem using these new constraints (i.e. the abundance variations), has reinvigorated the study of globular clusters, and at the same time has challenged our understanding of nucleosynthesis and stellar evolution. Several scenarios have been put forward to explain the presence of multiple stellar populations in globular clusters, nearly all requiring multiple generations of stars. The basic hypothesis in these models is that a second generation of stars is born during the early life of the globular cluster from the chemically-processed ejecta of some first generation stars in order to account for the signature multiple stellar populations observed in old globular clusters today. Many of these scenarios are mutually exclusive. Therefore, to determine which of them fits the current evidence the best became the priority of globular cluster studies. Modern observational facilities cannot resolve the globular cluster formation process in the early Universe. However, none of the scenarios for the origin of globular cluster and their multiple stellar populations make any distinctions between star/cluster formation at the present day and earlier epochs of the universe. Accordingly, the processes invoked in these scenarios can, in principle, be constrained by studies of the formation of young massive star clusters in the local Universe, which have similar sizes and masses as present-day globular clusters, but are significantly younger. In this work, I present some of the strongest constraints from such studies coming from the gas content of young massive clusters and their star formation histories. These studies showed that: 1) young massive clusters are consistent with a single star formation burst, and 2) there is no significant cool gas reservoirs left within young massive clusters that can fuel future star-formation events. These results are in stark contrast with the predictions of nearly all the scenarios that have been proposed to explain the origin of abundance variations in globular cluster stars, which require that young massive clusters should host multiple star formation events.
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