Binaries in globular clusters

Abstract

Recent observations have shown that globular clusters contain a significant binary population. This is a dramatic change from the conventional view of even a decade ago, which held that globular clusters formed without any binaries at all, since the observed X-ray binaries were understood to be formed through dynamical capture. Over the last few years, a number of different observational techniques have resulted in the detection of a substantial number of binaries most of which are believed to be primordial. When the many selection effects are taken into account, these detections translate into a binary abundance in globular clusters that may be somewhat smaller than those in the Galactic disk and halo, but not by a large factor. Within the current uncertainties, it is even possible that the primoridal binary abundance in globular clusters is comparable to the Galactic disk. We discuss different successful optical search techniques, based on radial-velocity variables, photometric variables, and the positions of stars in the color-magnitude diagram. In addition, we review searches in other wavelengths, which have turned up low-mass X-ray binaries and more recently a variety of radio pulsars. On the theoretical side, we give an overview of the different physical mechanisms through which individual binaries evolve. We discuss the various simulation techniques which recently have been employed to study the effects of a primordial binary population, and the fascinating interplay between stellar evolution and stellar dynamics which derive globular cluster evolution.