Formation of compact binaries in globular clusters

Globular clusters are overabundant per unit mass in neutron-star binaries, including low-mass X-ray binaries and radio pulsars. We report here on two complementary population synthesis studies which relate directly to the formation and evolution of neutron star binaries in globular clusters. In the first we consider an early population of massive stars, including those in binary systems, which produce the original population of neutron stars. We compute the probability of retention of these neutron stars, and quantitatively confirm the idea that the retention fraction for neutron stars born in binary systems is greatly enhanced over those born in isolated stars. However, with the currently fashionable natal kick velocities given to neutron stars, the retention fraction may well be insufficient to explain the current population of neutron star binaries. In the second study, we follow a large population of primordial binaries and neutron stars throughout the lifetime of a globular cluster whose properties may be similar to 47 Tuc. We directly compute all 3-body interactions among binary systems, neutron stars, and isolated field stars throughout the history of the cluster. The evolution of certain types of neutron star binaries is followed up to the current epoch. The numbers of close, recycled, binary radio pulsars are evaluated and compared with the results of radio observations. We find that for an initial population of some 10(6) primordial binaries and similar to 10(4) neutron stars, we can plausibly account for the large numbers of binary radio pulsars that are inferred to exist in clusters such as 47 Tuc.