COLLISIONS AND CLOSE ENCOUNTERS BETWEEN MASSIVE MAIN-SEQUENCE STARS

Collisons and close encounters between two massive (1 less than or sim ilar M/M. less than or similar 100) main-sequence stars have been stud ied using smooth-particle hydrodynamics (SPH). The stars are represent ed by Eddington standard models, which have the density profile of a p olytrope with n = 3 but mass-dependent binding energy and adiabatic in dex 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 calculatio ns 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 enc ountered in dense stellar systems such as galactic nuclei. We give app roximate scaling relations and fitting formulae for the amount of mass loss and for the critical impact parameters for capture or merging. T he more massive stars, which have smaller ratios of specific binding e nergy to the square of escape velocity, are more easily disrupted in c ollisions. In the limit of small relative velocity, our results for th e tidal capture radius agree closely with those of linear perturbation theory, although some nonlinear effects are always apparent. As the r elative velocity increases, the orbital energy of the colliding stars can only be dissipated by shock heating, and the critical capture radi us decreases much faster than predicted by linear theory. We also calc ulate cross sections and rates of stellar capture, merging, and mass l oss in a dense star cluster. We find that the average fractional mass loss per collision in a cluster does not depend sensitively on the ste llar velocity dispersion. Even when the velocity dispersion is as larg e as several times the typical escape velocity from a star, collisions are not very disruptive on the average, with only a few percent of th e mass liberated per collision. Our results should be useful for futur e dynamical studies of dense stellar systems incorporating the effects of stellar collisions and close dissipative encounters.