Star cluster ecology III: Runaway collisions in young compact star clusters

The evolution of young compact star clusters is studied using N -body simulations in which both stellar evolution and physical collisions between stars are taken into account. The initial conditions are chosen to represent R 136, a compact star cluster in the 30Doradus region of the Large Magellanic Cloud. The present runs do not include the effects of primordial binaries. We find that physical collisions between stars in these models are frequent, and that the evolution of the most massive stars and the dynamical evolution of the cluster are closely coupled. In all cases, a single star grows steadily in mass through mergers with other stars, forming a very massive ( > ∼ 100M⊙) star in less than 3–4 Myr. The growth rate of this runaway merger is much larger than estimates based on simple cross-section arguments, mainly because the star is typically found in the core and tends to form binaries with other massive stars there. The runaway is “rejuvenated” by each new collision, and its lifetime is extended considerably as a consequence. Observationally, such a star will appear in the Hertzsprung-Russell diagram as a blue straggler. When the runaway forms a black hole, the binary in which it is found is usually dissociated. We further investigate the sensitivity of the runaway to different formulations of mass loss from high-mass main sequence stars. We find that, while the runaway process is less pronounced in the presence of strong stellar winds, the basic effect persists even in the face of large mass loss.