A search for cataclysmic variables in globular clusters
Abstract
This thesis is a multiwavelength search for cataclysmic variables (CVs) in globular star clusters. Its aim is to test models which predict that significant numbers of these binary-stars should form via tidal capture or exchange collisions in clusters with dense cores. Its principal focus is on two nearby clusters, NGC 6397 and NGC 6752, which are so-called 'post-core-collapse' clusters with highly dense cores. Complementary observing programs were carried out at optical and x-ray wavelengths, using the Hubble Space Telescope (HST), ground-based optical telescopes, and the ROSAT x-ray satellite. The optical studies, aimed at detecting CVs in quiescence, are photometric studies whose primary selection criterion is for objects with H alpha emission lines. Two x-ray sources toward omega Cen (NGC 5139) at radii of greater than 10 arcminutes, which had previously been considered potential cluster CVs, are identified as foreground dMe stars, reinforcing the view that a significant fraction of x-ray sources at large radii are not cluster members. Candidate CVs are identified in both NGC 6397 and NGC 6752. ROSAT High Resolution Imager observations reveal 3-5 faint x-ray sources less than 1.5 arcminutes from the centers of each cluster, whose luminosities are typical of galactic CVs. HST Planetary Camera studies, coupled with analyses of archival HST Faint Object Camera images, yielded three candidates within 10 arcseconds of the center of NGC 6397 which are both H alpha bright and UV-bright. Associating these optical candidates with x-ray sources found with ROSAT produces a self-consistent picture of these stars as CVs with properties similar to those of CVs known elsewhere in the galaxy. Ground-based observations yielded two CV candidates in NGC 6752 at radii of 2-3 arcminutes. If the x-ray sources identified near the centers of NGC 6397 and NGC 6752 are CVs, the total CV population in the two clusters is likely to be greater than 10-20 and greater than 20-30, respectively. The total number of CVs in NGC 6397 inferred from the optical candidates agrees to within a factor of two. These inferred total populations are in reasonable agreement with theoretical predictions of CV formation rates.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1994
- DOI:
- 10.1080/00107519408222131
- Bibcode:
- 1994PhDT.........1C
- Keywords:
-
- Astronomical Photometry;
- Cataclysmic Variables;
- Globular Clusters;
- H Alpha Line;
- Star Distribution;
- X Ray Astronomy;
- X Ray Sources;
- Emission Spectra;
- Hubble Space Telescope;
- Rosat Mission;
- Space Observations (From Earth);
- Stellar Luminosity;
- Astronomy