Encounters between binary and single stars and their effect on the dynamical evolution of stellar systems

Abstract

More than 1.4times10$sup 4$ computer-simulated encounters between binaries and field stars have been run for various preencounter relative velocities V/subf/ and impact parameters p. At low values of V/subf/, the encounter causes the binary orbit to shrink and become more tightly bound with the two more massive of the three stars involved in the encounter remaining in the binary and the third being ejected from the binary at a relatively high velocity. As V/subf/ increases, the amount of orbit shrinking decreases until, for V/subf/ greater than a well-defined velocity V/subE/, the encounter tends to increase the semimajor axis of the binary and eventually to break it apart. We find that about 10% of the binaries with semimajor axes a$sub 0$>10$sup 3$ au have broken apart in the solar neighborhood. In globular clusters almost all primordial binaries with a$sub 0$>35 au have broken apart; this is true for binaries with a$sub 0$>100--1000 au in open clusters and a$sub 0$>1--2 au in galactic nuclei. The breakup of these loosely bound binaries has relatively little effect on the dynamical evolution of the stellar system. However, the shrinking of the more tightly bound binary orbits, which feeds kinetic energy into the cluster, will destroymore » an open cluster in a few times 10$sup 8$ y unless it is either unusually massive or unusually deficient in binaries. While the lifetime of a globular cluster due to this mechanism is about 10$sup 11$ y, it is likely that binary stars limit the maximum density attainable in the nucleus of a globular cluster. The presence of the binaries transforms the familiar instability leading to a rapid shrinking of the core of the cluster and to the buildup of a high-density central cusp into one which causes an accelerated rate of decrease in the semimajor axes of the binaries in the core. Binaries are not likely to have any significant effect on the dynamical evolution of a dense galactic nucleus.« less