Collaborative Research: Rethinking the Fundamentals of Massive Star Clusters

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A Globular Cluster is a tight group of hundreds of thousands of stars bound together by gravity. There are hundreds of Globular Clusters orbiting around our own Milky Way galaxy. Globular Clusters serve as fossils that trace conditions in the universe at the earliest times, when galaxies were just starting to form. Intriguing differences are observed between individual Globular Clusters, which provide clues about the formation of our Galaxy. In Globular Cluster stars, the concentrations of atoms belonging to different chemical elements vary in ways that challenge conventional wisdom about the ages, origins and life cycles of stars. The investigators will make observations with some of the world?s largest and best-equipped telescopes to test theoretical models of how stars, star clusters and galaxies come to be.The principal work involves (a) developing a unified scenario for self-enrichment among star clusters; (b) understanding the stellar content of Globular Clusters through novel dynamical and stellar population investigations; and (c) studying the relationship between Globular Clusters and galaxies at the extreme upper limit of the Globular Cluster mass function. New optical and near-infrared photometric and spectroscopic observations will be made to explore how measurable quantities (such as mass-to-light ratios and abundance anomalies) depend on age, metallicity, stellar initial mass function, and dynamical evolution. Intriguing trends, many of which confound standard interpretations, have already been identified: 1) self-enrichment appears to be implicated in abundance spreads in Galactic Globular Clusters and in a mass-metallicity relation found in extragalactic Globular Cluster systems; 2) observed properties of galaxy stellar halos, and initial measurements of gas in intermediate-age clusters, each challenge the favored mechanism for self-enrichment; 3) the dependence of mass-to-light ratio on metallicity is opposite to that expected from stellar evolution models; 4) an expanding zoo of compact stellar systems has been identified, including ultra compact dwarfs, that could be formed either by tidal stripping of dwarf galaxies or from Globular Cluster mergers; 5) the elevated mass-to-light ratios of ultra compact dwarfs may be explained by massive central black holes. The investigators will clarify each of these issues, test stellar evolution and population synthesis models, and trace the origins of compact stellar systems.Since Globular Clusters are more complex than formerly realized, understanding their properties and how they relate to other types of stellar systems has wide implications in astrophysics; inferences drawn from Globular Clusters inform theories of the formation and evolution of stars, galaxies, black holes, and dark matter. This fundamental work impacts forefront research from star formation to cosmology. To promote the broader impacts of this research effectively, the partnership with the non-profit organization, ?Science Buddies?, will be continued. This highly successful pilot program has already engaged thousands of K-12 students from diverse backgrounds, including disadvantaged groups. A vigorous program of professional and public outreach will continue, with a proven record of including women, undergraduates, and minorities in forefront research. A successful pilot program to involve undergraduates in Massive Clusters research will be expanded. Students and post-docs will receive excellent training with access to world-class facilities and to senior researchers, participating in all aspects of the research process. A user-friendly, legacy catalog of newly discovered compact stellar systems and their fundamental properties will be publicly available online.