Spectroscopic Identification of Probable Cataclysmic Variables in the Globular Cluster NGC 6397

Abstract

The existence of compact binaries in dense clusters is of considerable interest for testing theories of binary production and evolution, as well as the dynamical evolution and survival of the clusters in which they reside. Here, using the newly repaired Hubble Space Telescope to observe the collapsed core of the nearby globular cluster NGC 6397, we report the first spectra of stars well below the main-sequence turnoff near the center of a dense globular star cluster. The spectra confirm our WFPC1 photometry that these objects are almost certainly the long-sought population of cataclysmic variables (CVs) in globulars, and that they are the likely counterparts for the recently discovered (ROSAT) low-luminosity X-ray sources in this cluster. The spectra suggest the CVs are magnetic (DQ Her type), raising the intriguing possibility that magnetic white dwarfs are preferentially produced in globular cluster cores.