Binary--Single Star Interactions in Globular Clusters

An extensive series of three-body interactions involving hard binaries and single stars are calculated by direct integration of ∼105 encounters. Unlike previous calculations, the stars have different mass ratios, chosen to be representative of the stellar and remnant population in a globular cluster, and most effort was devoted to the difficult case of hard binaries, the most likely primordial binaries to survive in a cluster. Graphs, tables and analytical fits to differential and integral cross sections for properties of different interaction channels are presented and discussed. The properties include exchange, binary semi-major axis, eccentricity, and vector angular momentum, binary recoil velocity, and dissipative encounters (collision or tidal capture). We compute mean energy transfer as a function of the field star velocity and binary properties (relevant for Fokker-Planck models of the evolution of clusters containing binaries). We find that mass-ratios of order 2 lead to significant differences and new phenomena in the interactions compared with the equal mass case. Resonances are followed and are found to contribute strongly to the cross sections for physical collisions of stars. We find that for moderately hard binaries, exchange of heavy field stars is the dominant process. After such an exchange the physical cross section for subsequent interactions increases, even though the binary is hardened (binding energy increased) by the interaction. For very hard binaries, dissipative interactions dominate the cross section.