THE EVOLUTION OF A PRIMORDIAL BINARY POPULATION IN A GLOBULAR-CLUSTER

We present a simple model for the evolution of a primordial binary population in a globular cluster. Binaries are characterized by a two-dimensional distribution function in binding energy and distance to the cluster center (or, equivalently, internal and external orbital energy). The model forms a middle ground between realistic N-body calculations and analytic estimates based on average amounts of binary hardening per scattering event. We present a series of Monte Carlo simulations for an initial population of 5 x 10(4) binaries against a fixed background population of 5 x 10(5) single stars in a tidally truncated cluster model. We follow the individual histories of all binaries as they experience a variety of different physical mechanisms: mass segregation, scattering recoil, escape from the cluster, and, optionally, coalescence through gravitational radiation losses and collisional mergers. The main observational consequences of our simulations are (1) most binaries are destroyed by binary-binary interactions. In the point-mass approximation, the rest escape. In a more realistic model, the majority of the rest merge. (2) At any instant, most of the remaining binaries are drifting in toward the center, before their first strong encounter. (3) A typical binary spends most of its active life (after its first strong scattering event) in or near the cluster core. (4) The few binaries which receive a recoil sufficient to place them in the halo past the half-mass radius remain there long enough to make a significant contribution to the radial binary distribution. (5) This latter effect is strongly suppressed by collisions and spiral-in, both of which tend to lower the average distance of a binary from the cluster center. (6) In conclusion, the resulting binary distribution exhibits appreciable correlation between binding energy and energy of center-of-mass motion, and at no time comes close to a multimass King model. Therefore, a dynamical model is imperative for any meaningful comparison of a theoretical binary distribution with observations.