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Session 98 - Globular Clusters.
Oral session, Friday, January 09

[98.02] Stellar Collisions: Probing Dynamics and Populations in Globular Clusters

A. Sills (Yale U.)

Globular clusters have long been used as laboratories for two distinct subfields of astronomy: stellar evolution and stellar dynamics. Evolutionists treat a globular cluster as a large population of stars, all with the same distance, age and composition but of different masses. In comparison, a globular cluster, to the dynamicist, is a system of about 10^5 point masses which are interacting only under the influence of their mutual gravity. However, it is becoming clear that stellar evolution and stellar dynamics cannot be treated as independent processes in globular clusters. In order to completely model globular clusters, we must consider the interactions between stellar evolution and stellar dynamics. One particularly useful interaction appears in the study of blue stragglers. Blue stragglers are main sequence stars which are more massive than the stars at the turnoff of the cluster, and were probably formed through the merger of two lower-mass stars. If we assume that blue stragglers are formed by direct stellar collisions (possibly mediated by binary stars), the distribution of blue stragglers in the Hertzsprung-Russell diagram and the total number of blue stragglers will constrain the dynamical properties of the globular cluster and give us information about the stellar populations in the cluster. My work has been concerned with creating theoretical distributions of blue stragglers to compare with observational results. These distributions depend on the dynamical properties of the cluster, such as density and velocity dispersion, and on information about the populations present in the cluster, such as the binary fraction, the distribution of binary semi-major axes, the mass function, and the details of the evolution of the collision products. This work begins to bridge the gap between the different ways of modeling globular clusters. By calculating distributions of a particular collision product, blue stragglers, we can not only learn about blue stragglers themselves, but are also able to study the dynamical state of globular clusters, and constrain the binary population in globular clusters.

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