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 cont ained 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 bina ries were set to 60 R. (0.28 AU). We considered intruders of three mas ses: 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-d imensional 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 th e results of the SPH runs, we computed the cross sections for clean ex change, denoted by sigma(cx); the formation of a triple system, denote d by sigma(trp); and the formation of a merged binary with an object f ormed from the merger of two of the stars left in orbit around the thi rd star, denoted by sigma(mb). For encounters between either binary an d a 1.4 M. neutron star, sigma(cx) approximately 0.7sigma(ex) and sigm a(mb) + sigma(trp) approximately 0.3sigma(ex). For encounters between either binary and the 0.8 M. main-sequence star, sigma(cx) approximate ly 0.50ex and sigma(mb) + sigma(trp) approximately 1.0sigma(ex). If th e 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) + si gma(trp) approximately 0.8sigma(ex). For encounters between either bin ary 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 dw arf is replaced by a main-sequence star of the same mass, we have sigm a(cx) approximately 0.5sigma(ex) and sigma(mb) + sigma(trp) approximat ely 1.6sigma(ex). Although the exchange cross section is a sensitive f unction of intruder mass, we see that the cross section to produce mer ged binaries is roughly independent of intruder mass. The merged binar ies produced have semi-major axes much larger than either those of the original binaries or those of binaries produced in clean exchanges. C oupled with their lower kick velocities, received from the encounters, their larger size will enhance their cross section, shortening the wa iting time to a subseqent encounter with another single star.