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A.M. Cool (San Francisco State U.), P.D. Edmonds, J. E. Grindlay (Harvard U.)
We present photometric and spectroscopic HST observations of cataclysmic variables and a new class of faint ultraviolet (UV) stars in the collapsed-core globular cluster NGC 6397. Color-magnitude diagrams constructed from UBVI images obtained with the WFPC2 reveal seven UV-bright stars that lie between the main sequence and the white dwarf sequence in a U vs. U-B CMD. Four of these are cataclysmic variables (CVs), whose spectra show the emission lines and whose lightcurves show the "flicker" characteristic of accreting binaries. All four have quite faint disks as compared to field CVs, which may imply low accretion rates. Unlike the CVs, the other three UV-bright stars show no photometric variability and have broad-band colors characteristic of B stars. An FOS spectrum we have obtained of one of these non-flickering (NF) stars reveals a broad H-beta line in absorption. Detailed comparisons with stellar atmosphere models yield log g = 6.25 +/- 1.0 and an effective temperature of 17,500 +/- 5,000 K. Using these line parameters and the luminosity of the NF we show that the spectrum is consistent with a helium white dwarf with a mass of about 0.25 solar masses and an age of 0.1-0.5 Gyr. The NF spectrum is significantly Doppler shifted from the expected wavelength, which may imply the presence of a dark, massive companion. We suggest that such low-mass He WDs could be 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. Both the CVs and the possible He WDs are strongly concentrated toward the cluster center, to the extent that mass segregation from 2-body relaxation alone may be unable to explain their distribution.
The author(s) of this abstract have provided an email address for comments about the abstract: cool@sfsu.edu