Testing for binary companions among rare stellar remnants in the star cluster NGC 6752

By: Yarik Patel

Department: Physics

Faculty Advisor: Dr. Adrienne Cool

White dwarfs composed of helium are rare stellar remnants that would take more than the age of universe time to form via single star evolution. At present they are thought to require an external process to form: mass transfer within a gravitationally bound pair of stars--a binary star system. In globular star clusters, binaries stars play a vital role in stirring up the cluster and prolonging the possible formation of a massive black hole at the center. For understanding the dynamics of clusters, we must study binaries. A set of recently identified helium white dwarfs (He WDs) candidates provide a new way to do so. Because white dwarfs are too faint to see with ground-based telescopes, we use the

Advanced Camera for Surveys on the Hubble Space Telescope. We estimate the masses of the candidate He WDs by comparing their observed colors and brightnesses to theoretical models, finding a typical mass of about 0.3 Msun. We then examine the radial distribution of the He WDs within the cluster. Using the fact that higher-mass stars are concentrated to the cluster center while lower-mass stars are more spread out due to cluster dynamics, we derive an independent mass estimate, finding an average value of least 0.6-0.7 Msun. That the dynamically measured masses exceed those determined from brightness and color strongly suggests the presence of unseen binary companions. This confirms that these He WDs are likely to be members of binary systems in NGC 6752. Further study of these rare stellar remnants, including estimates of their ages, will provide insight into the characteristics of the primordial binaries from which they formed. This in turn will help to constrain computer simulations of globular clusters to further our understanding of cluster dynamics.