Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...441..617s&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 441, no. 2, p. 617-628
Astronomy and Astrophysics
Astronomy
29
Cataclysmic Variables, Eclipsing Binary Stars, Globular Clusters, Image Analysis, Spaceborne Astronomy, Color-Magnitude Diagram, Faint Object Camera, Hubble Space Telescope, Light Curve, Main Sequence Stars, Stellar Luminosity, Stellar Spectrophotometry
Scientific paper
The cores of globular clusters have been predicted to be rich in close, interacting binaries. We have used the Hubble Space Telescope (HST) to test this prediction by monitoring 730 stars in the core of NGC 5752 for rapid variability. Thirty-one Faint Object Camera (FOC) images at 2200 A, with time resolution of 11 minutes, and spanning 7 hr have been searched for variables. Artificial star tests demonstrate that we should have found virtually all variables brighter than m2200 = 20 (i.e., M220 less than or = 6.75) with variability amplitude Delta m greater than or = 0.25 mag and most variables brighter than m2200 = 22 (i.e., M2200 less than or = 8.75) with Delta m greater than or = 0.35 mag. No variables were found; eight were expected based on tidal capture models in the one-fourth of the core that we observed. Our results strongly constrain the number of cataclysmic and contact binaries (with the above variability and luminosity limits) in the core of NGC 6752 to be less than or = 10 in total with 95% probability. A color-magnitude diagram reveals the presence of four UV-excess stars below the turnoff, all located within 6 arcsec of the cluster center. None show detectable variability (sigma less than or = 0.1 mag) during the 7 hr FOC observing sequence. Two of them are detected in archival HST planetary camera (PC) R and H-alpha images, but show no H-alpha excess. The lack of variability, the absence of H-alpha emission, as well as the fact that none of them appear in outburst in any of four epochs, strongly suggest that they are not cataclysmic variables. This is the strongest observational test to date of simple tidal capture theory. NGC 6752 appears to have significantly fewer tidal capture binaries than the simple tidal capture theory predicts. Five blue stragglers are discovered, as well as 14 horizontal-branch stars which completely dominate the cluster light at 2200 A. The main sequence luminosity function of NGC 6752 is remarkably flat at the cluster center. This is in contrast to the rising value found by Da Costa at 10.1 arcmin less than or = r less than 13.2 arcmin and Richer et al. at 6 arcmin less than or = r less than 10 arcmin. Mass segregation probably accounts for the difference. The preponderance of low-mass stars in the outer parts of NGC 6752 may be due to enrichment (i.e., depletion of the core and transport outward of these low-mass stars) rather than being primordial. If this is true, then the suggestion that dissolved globular cluster low-mass Population II stars may be an important component of the mass budget of the Galactic halo is further weakened.
Bergeron Louis E.
Drissen Laurent
Paresce Francesco
Shara Michael M.
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