The Relationship Between Mass and Multiple Populations in Globular Clusters and Their Streams
Samantha Usman
Graduate Student
University of Chicago
Globular clusters are often considered the ideal stellar system for comparison: they form at the same time and from the same cloud of gas, meaning each cluster is a pristine set of hundreds of thousands or millions stars that are the same age with the same composition. Except there is a complication: despite forming out of the same material, the stars in globular clusters do not have the same composition. They contain multiple populations: distinct groups of stars within a single cluster that exhibit variations in their chemical composition, particularly in elements like sodium, magnesium, aluminum and oxygen. These chemical patterns were first identified more than fifty years ago, but the mechanism for this enrichment is not well understood. There is one hint to understanding this phenomenon: more massive globular clusters have proportionally more enriched stars than less massive ones. We probe this relationship by observing globular clusters and their disrupted remnants called stellar streams. We quantify the composition of the stars in these stellar systems using spectroscopy from the Magellan telescope. We find that globular clusters need to have a mass of 10^5 solar masses at its time of formation to have multiple populations, while clusters that form with a mass of 10^6 solar masses have approximately 75% of their stars enriched.
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