On the incidence of close binary stars in globular clusters and the nature of the cluster X-ray sources

Abstract

{ '1§i7I~Ev'§€6I lie:-I Mon. Not. R. astr. Soc. (1977) 178, 335~343 On the incidence of close binary stars in globular clusters and the nature of the cluster X-ray sources Virginia * Department of Physics, University of California, Irvine, California 92 71 7, USA, and Astronomy Program, University of Maryland, College Park, Maryland 20 742, USA Received 1976 July 22; in original fonn 1976 May 17 Summary. Recent calculations by Hills suggest that the globular clusters could not have formed with more than 20 per cent of the normal Population I fraction of their stars in binary systems. The fact that the clusters have more than their fair share of novae and U Geminorum stars (three each out of ~ 200 of each known, while the clusters contain only about 10'“ of the mass and 10’3 of the luminosity of the galaxy) therefore becomes surprising. The hypo- thesis of binary capture within cluster cores suggested by Fabian, Pringle & Rees to account for the clusters’ high X-ray luminosity provides a few extra systems, but neither it nor any of the similar encounter or capture mechanisms suggested can account for the novae and U Gem stars, which remain puzzling. The numbers of Algol-type and WUMa eclipsing binaries predicted by these hypotheses do not conflict with data presently available, but careful searches for them would constitute a critical test of the theories. Background The identification of five strong, variable X-ray sources with the globular clusters NGC 1851, 6440, 6441, 6624 and 7078 (Giacconi et al. 1974; Clark, Markert & Li 1975; Markert et al. 1975) implies that the ratio of X-ray luminosity to optical luminosity (or mass) for the clusters is about 100 times that for our Galaxy as a whole (Katz 1975). The sorts of systems thought to be responsible for most galactic X-ray sources (neutron stars accreting matter supplied by normal companions in close binary systems) caxmot have survived in globular clusters from the time of their formation 10 billion or so years ago until the present (Clark 1975). Thus, some other mechanism is required. The clusters involved are richer and more centrally condensed (or, alternatively, have larger escape velocities and smaller core relaxation times) than typical globular clusters (Arp 1965; Kukarkin 1974; Peterson & King 1975), although not all rich, condensed clusters are X-ray sources (Uhner, Murray & Gursky 1976a). The mechanisms suggested have, there- fore, focused on processes that would be favoured by high stellar density and large escape * Alfred P. Sloan Foundation Research Fellow, 1972-74. © Royal Astronomical Society 0 Provided by the NASA Astrophysics Data System