Blue Stars and Binary Stars in NGC 6397: Case Study of a Collapsed-Core Globular Cluster

The dense central region of NGC 6397 contains three classes of stars whose origins are likely related to stellar interactions: blue stragglers, cataclysmic variables, and probable helium white dwarfs. We summarize results concerning CVs and HeWD candidates that have been identified in two imaging studies with Hubble Space Telescope, including one CV candidate that appears well outside the cluster core. We also present results concerning binaries containing two main sequence stars in the central parts of the cluster. Proper motion information derived from two epochs of HST data is used to remove field stars from the sample. Binaries are then identified on the basis of their positions in the color-magnitude diagram. We set an upper limit of about 3% on the fraction of main sequence stars with primary masses in the range 0.45-0.8 solar masses and mass ratios q >~ 0.45. Extrapolating to all mass ratios gives an estimated binary fraction of less than about 5%-7%. Even in these small numbers, such pairs are likely to be key players in the processes that give rise to the more exotic stellar populations.