Cataclysmic Variables and a New Class of Faint Ultraviolet Stars in the Globular Cluster NGC 6397

Abstract

We present evidence that the globular cluster NGC 6397 contains two distinct classes of centrally concentrated W-bright stars. Color-magnitude diagrams constructed from U(336), B(439), V(555), and I(814) data obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 reveal seven UV-bright stars fainter than the main-sequence turnoff, three of which had previously been identified as cataclysmic variables (CVs). Light curves of these stars show the characteristic "flicker" of CVs as well as longer term variability. A fourth star is identified as a CV candidate on the basis of its variability and UV excess. Three additional UV-bright stars show no photometric variability and have broadband colors characteristic of B stars. These nonflickering UV stars are too faint to be extended horizontal-branch stars. We suggest that they could be low-mass helium white dwarfs, formed when the evolution of a red giant is interrupted, due either to Roche lobe overflow onto a binary companion or to envelope ejection following a common-envelope phase in a tidal-capture binary. Alternatively, they could be very low mass core-He-burning stars. Both the CVs and the new class of faint UV stars are strongly concentrated toward the cluster center, to the extent that mass segregation from two-body relaxation alone may be unable to explain their distribution.