DISCOVERY OF CANDIDATE CATACLYSMIC VARIABLES IN THE POST-CORE-COLLAPSE GLOBULAR CLUSTER NGC 6397

摘要

A photometric search for objects with Hα emission lines 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 (Hα) filters, and photometry carried out on ～900 stars detected in the central ～38″ x 35″ of the cluster. Limiting magnitudes of R ～ 21 and 19.5 were reached in the F675W and F656N images, respectively. Three Hα-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 Hα emission-line strengths (EW[Hα]) approx > 15-26 A) make these objects good candidates to be cataclysmic variables (CVs) in this nearby, post-core-collapse cluster. All three Hα-bright objects are found to be UV-bright in a photometric comparison of the F675W images with archival HST Faint Object Camera 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 Hα-bright stars are within the error circles of three X-ray sources previously found with the R0SAT HRI and are likely to be the dominant source of the X-ray emission. The implied X-ray to optical flux ratios (f_x/f_v ～ 0.8-4.2) are consistent with CVs known elsewhere in the Galaxy. Two additional stars with possible Hα emission are also discussed, along with stars identified on the basis of UV emission. 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 approx > 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 agreement with previous estimates made on the basis of X-ray observations, and with theoretical predictions scaled to this cluster.