Collisions and close encounters between massive main-sequence stars

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Abstract
Collisons and close encounters between two massive (1 less than or similar M/M. less than or similar 100) main-sequence stars have been studied using smooth-particle hydrodynamics (SPH). The stars are represented by Eddington standard models, which have the density profile of a polytrope with n = 3 but mass-dependent binding energy and adiabatic index 4/3 < GAMMA1 < 5/3. The equation of state is that of an ideal gas plus thermal radiation. We have performed a large number of calculations to obtain extensive coverage of the parameter space. In particular, the stellar masses, relative velocity, and collision impact parameter are all varied over wide ranges, representative of the conditions encountered in dense stellar systems such as galactic nuclei. We give approximate scaling relations and fitting formulae for the amount of mass loss and for the critical impact parameters for capture or merging. The more massive stars, which have smaller ratios of specific binding energy to the square of escape velocity, are more easily disrupted in collisions. In the limit of small relative velocity, our results for the tidal capture radius agree closely with those of linear perturbation theory, although some nonlinear effects are always apparent. As the relative velocity increases, the orbital energy of the colliding stars can only be dissipated by shock heating, and the critical capture radius decreases much faster than predicted by linear theory. We also calculate cross sections and rates of stellar capture, merging, and mass loss in a dense star cluster. We find that the average fractional mass loss per collision in a cluster does not depend sensitively on the stellar velocity dispersion. Even when the velocity dispersion is as large as several times the typical escape velocity from a star, collisions are not very disruptive on the average, with only a few percent of the mass liberated per collision. Our results should be useful for future dynamical studies of dense stellar systems incorporating the effects of stellar collisions and close dissipative encounters.

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