Main-sequence binaries, contact binaries, and blue stragglers in globular clusters

Abstract

The existence of binary stars in globular clusters has a long history of controversy1–3. At present the only evidence for binaries in globular clusters comes from binaries in extreme states: cataclysmic variables4,5 and X-ray sources (see refs 6 and 7 for reviews, and ref. 8 for new observations of very weak X-ray sources). The existence of the latter is customarily ascribed to tidal capture of red dwarf companions by neutron stars (or possibly solar-mass black holes)9. In fact, tidal capture should be at least as effective in creating close binaries comprising two main-sequence stars9,10. Subsequent evolution of such binaries is very likely to lead to mass exchange between the two, and possibly merger. In conventional calculations, gravitational radiation drives the binary orbital evolution11,12; recently, it has been shown that in the denser globular clusters, where tidal capture is most effective, gravitational encounters with passing field stars also have an important role, both by slow, cumulative compression of the binary orbit, and by occasional catastrophic physical collisions13. On the basis of these mechanisms, we predict that globular clusters should contain substantial numbers of: close binaries; low-mass contact binaries, perhaps similar to the somewhat more massive WUMa stars; and ‘blue stragglers’, stars lying on the extrapolated main sequence beyond the turn-off. These objects should be most common in the densest regions of the Galaxy's globular cluster system.