Helium White Dwarfs and BY Draconis Binaries in the Globular Cluster NGC 6397

Abstract

We examine 15 orbits of Hubble Space Telescope observations of the nearby post-core-collapsed globular cluster NGC 6397 using narrowband photometry in Hα with broadband photometry in V, R, and I. We recover the candidate helium white dwarfs (He WDs) recently discovered by Cool et al., of which one was examined spectroscopically by Edmonds et al., along with a sequence of these objects extending down to the magnitude limit of the survey. We also find a sequence of stars with weak Hα emission and magnitudes characteristic of BY Draconis stars; their offset above the main sequence also suggests they are probable binaries. The radial distributions for both the He WD and BY Dra candidates are significantly more centrally concentrated than the main-sequence stars, suggesting that indeed both populations are in binary systems. Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555.