Cataclysmic Variables and a Candidate Helium White Dwarf in the Globular Cluster NGC 6397

Abstract

We have used HST/FOS to study faint UV stars in the core of the nearby globular cluster NGC 6397. We confirm the presence of a 4th cataclysmic variable (CV) in NGC 6397 (CV 4), and we use the photometry of Cool et al. (1998) to present evidence that CVs 1--4 all have faint disks and probably low accretion rates. By combining these results with new UV spectra of CV 1 and the published spectra of Grindlay et al. (1995) we present new evidence that CVs 1--3 may be DQ Her systems, and we show that CV 4 may either be a dwarf nova or another magnetic system. Another possibility is that the CVs could be old novae in hibernation between nova eruptions. We also present the first spectrum of a member of a new class of UV bright stars in NGC 6397. These faint, hot stars do not vary, unlike the CVs, and are thus denoted as ``non-flickerers'' (NFs). Like the CVs, their spatial concentration is strongly concentrated toward the cluster center. Using stellar atmosphere models we have determined log g = 6.25, and T_eff = 17,500 K for this NF. Using these line parameters and the luminosity of the NF we show that the NF spectrum is consistent with a helium WD having a mass of ~0.25 solar masses and an age between 0.1 and 0.5 Gyr (depending on the models used). The NF spectrum appears to be significantly Doppler shifted from the expected wavelength, suggesting the presence of a dark, massive companion, probably a carbon-oxygen WD.