Production of Recycled Pulsars in Globular Clusters Via Two-Body Tidal Capture

We investigate the numbers of binary systems containing a neutron star that are formed in the cores of globular clusters via two-body tidal capture. We also study the subsequent evolution of the resulting binary systems. Particular attention is paid to the effects of the stripping of mass from the captured star during the capture process. We point out that in some cases it may be possible for mass ejection to be an important complement to tidal dissipation in producing a bound orbit and in circularizing the orbit.Our basic approach is to follow the histories of many neutron stars in the core of a model globular cluster, using Monte Carlo techniques. These simulations allow us to explore various scenarios for mass stripping during capture and orbital circularization, and the effects of magnetic braking on the circularized orbits. Calculations are carried out for two model clusters: 47 Tuc, which has a dense core, and the relatively more diffuse cluster omega-Cen. Distributions of orbital periods for newly formed binaries containing a neutron star in orbit with either a main-sequence star or a giant are presented. Each binary is followed until contact between the captured star and the neutron star-i.e., Roche lobe overflow from the extended star-is established. When contact is established through orbital decay via either gravitational radiation or magnetic braking, we do not follow the system further because of present uncertainties in the subsequent binary evolution. We argue, however, that these systems should be regarded as potential progenitors of "recycled" (spun-up) pulsars. When contact is established through the expansion of a giant which comes to fill its Roche lobe as a result of its own nuclear evolution, we do follow the evolution of the system through mass transfer and to the final state, which is a wide-orbit system with a recycled pulsar. Such systems may subsequently undergo an ionization or exchange collision with another cluster star, whereby the neutron star (or its companion) is liberated. Orbital period distributions for the surviving wide-orbit systems are presented, together with the period distributions of binary systems that come into contact as the result of a dissipative process.Among our results, we find that there should be approximately 90N(Tuc) and approximately 28N(Cen) recycled pulsars in 47 Tuc and omega-Cen, respectively, due to two-body tidal capture. The quantities N(Tuc) and N(Cen) are scaling parameters defined as the numbers of neutron stars in the cluster, divided by 10(3) in the case of 47 Tuc and by 10(4) in the case of omega-Cen. The numbers of wide binary millisecond pulsars are somewhat sensitive to the choice of magnetic braking and mass stripping parameters. The median numbers of such systems among the different simulations are 15N(Tuc) and 2.5N(Cen) respectively, in the two clusters. Based on simple scaling arguments, we find that two-body tidal capture should have produced approximately 2000N(Tuc) or approximately 3000N(Cen), recycled pulsars in all of the Galactic globular clusters. These numbers would not be significantly diminished by eliminating all contributions from the capture of stars which are substantially evolved at the time of capture-i.e., our results would seem to be valid even if almost all giants with well-developed cores (i.e., core mass greater than or similar to O.15 M.) are destroyed by the capture.Other mechanisms, such as the capture of neutron stars by primordial binaries and accretion-induced collapse of tidally captured white dwarfs, are not considered. However, our results serve to quantify the effects of a process that is expected to play an important role, especially in the more centrally condensed clusters.