Discovery of Candidate Cataclysmic Variables in the Post--Core-Collapse Globular Cluster NGC 6397

Abstract

A photometric search for objects with H alpha emission in the globular cluster NGC 6397 has been carried out with the Hubble Space Telescope (HST) Planetary Camera. Images were obtained through the F675W (similar to Johnson R) and F656N (H alpha) filters, and photometry carried out on approximately 900 stars detected in the central approximately 38 x 35 arcseconds of the cluster. Limiting magnitudes of R approximately 21 and 19.5 were reached in the F675W and F656N images, respectively. Three H alpha-bright stars are found which have apparent magnitudes in the range R = 17.8-19.5. The corresponding absolute magnitudes (M_R = 5.6-7.3) and inferred H alpha emission-line strengths (EW(H alpha) greater than or approximately equal to 15-26 A) make these objects good candidates to be cataclysmic variables (CVs) in this nearby, post-collapse cluster. All three H alpha-bright objects are found to be UV-bright in a photometric comparison of the F675W images with archival HST Faint Object Camera (FOC) images of the cluster center through the F220W and F346M filters. Such UV excesses further support the identification of these objects as candidates to be CVs in the cluster. The H alpha-bright stars are within the error circles of three X-ray sources previously found with the ROSAT High Resolution Imager (HRI) and are likley to be the dominant source of the X-ray emission. The implied X-Ray to optical flux ratios (f_x/f_v approximately 0.8-4.2) are consistent with CVs known elsewhere in the Galaxy. Two additional stars with possible H alpha emission are also discussed, along the stars identified on the basis of UV emisssion. Preliminary completeness estimates suggest that these observations should be sensitive to approximately one-third to two-thirds of most varieties of CVs in the cluster, and that therefore greater than or approximately 5-10 CVs are likely to be present in the observed section of NGC 6397. Scaling these numbers to the cluster as a whole would increase these numbers by up to a factor of 2. A total CV population on the order of 5-20 is in reasonable with previous estimates made on the basis of X-ray observations, and with theoretical predictions scaled to this cluster.