Dynamics and Interactions of Binaries and Neutron Stars in Globular Clusters

Abstract

We model the dynamics of test binaries in isotropic, multimass models of Galactic globular clusters. The evolution of binary orbits through the cluster potentials is modeled, including second-order diffusion terms, and probabilities for close encounters with field stars are calculated. We carry out Monte Carlo simulations of the effects of the binary-single star encounters on the binary population and distribution in the cluster, and estimate the collision rate for different stellar populations in globular clusters with different structural parameters. In particular, we consider the rate for neutron stars and massive white dwarfs to undergo mass transfer due to collisional encounters, and hence estimate the population of observable millisecond pulsars in different clusters formed by binary-single star encounters. Assuming a Salpeter IMF, for low concentration clusters the core encounter rate is dominated by turnoff mass main-sequence stars and medium mass white dwarfs. For high concentration, high- density clusters the encounter probabilities are increasingly dominated by neutron stars and heavy white dwarfs. Hence we predict a smaller ratio of blue stragglers and cataclysmic variables to pulsars in high concentration clusters. The total number of millisecond pulsars, and the ratio of single to binary pulsars, is broadly consistent with the observed population, suggesting the binary-single star encounters contribute significantly to the pulsar formation rate in globular clusters, for the whole range of globular cluster types. The number of millisecond pulsars and the ratio of pulsars in different globular clusters is best explained by a total binary fraction comparable to that of the Galaxy, and a modest number of primordial neutron stars in the globular clusters.