CLOSE ENCOUNTERS OF THE 3RD-BODY KIND

We simulated encounters involving binaries of two eccentricities: e = 0 (i.e., circular binaries) and e = 0.5. In both cases the binary contained a point mass of 1.4 M. (i.e., a neutron star) and an 0.8 M. main-sequence star modeled as a polytrope. The semimajor axes of both binaries were set to 60 R. (0.28 AU). We considered intruders of three masses: 1.4 M. (a neutron star), 0.8 M. (a main-sequence star or a higher mass white dwarf), and 0.64 M. (a more typical mass white dwarf). Our strategy was to perform a large number (40,000) of encounters using a three-body code, then to rerun a small number of cases with a three-dimensional smoothed particle hydrodynamics (SPH) code to determine the importance of hydrodynamical effects. Using the results of the three-body runs, we computed the exchange cross sections, sigma(ex). From the results of the SPH runs, we computed the cross sections for clean exchange, denoted by sigma(cx); the formation of a triple system, denoted by sigma(trp); and the formation of a merged binary with an object formed from the merger of two of the stars left in orbit around the third star, denoted by sigma(mb). For encounters between either binary and a 1.4 M. neutron star, sigma(cx) approximately 0.7sigma(ex) and sigma(mb) + sigma(trp) approximately 0.3sigma(ex). For encounters between either binary and the 0.8 M. main-sequence star, sigma(cx) approximately 0.50ex and sigma(mb) + sigma(trp) approximately 1.0sigma(ex). If the main sequence star is replaced by a white dwarf intruder of the same mass, we have sigma(cx) approximately 0.6sigma(ex) and sigma(mb) + sigma(trp) approximately 0.8sigma(ex). For encounters between either binary and the 0.64 M. white dwarf, sigma(cx) approximately 0.6sigma(ex) and sigma(mb) + sigma(trp) approximately 1.3sigma(ex). If the white dwarf is replaced by a main-sequence star of the same mass, we have sigma(cx) approximately 0.5sigma(ex) and sigma(mb) + sigma(trp) approximately 1.6sigma(ex). Although the exchange cross section is a sensitive function of intruder mass, we see that the cross section to produce merged binaries is roughly independent of intruder mass. The merged binaries produced have semi-major axes much larger than either those of the original binaries or those of binaries produced in clean exchanges. Coupled with their lower kick velocities, received from the encounters, their larger size will enhance their cross section, shortening the waiting time to a subseqent encounter with another single star.