On the onset of runaway stellar collisions in dense star clusters -- II. Hydrodynamics of three-body interactions

Abstract

The onset of runaway stellar collisions in young star clusters is more likely to initiate with an encounter between a binary and a third star than between two single stars. Using the initial conditions of such three-star encounters from direct N-body simulations, we model the resulting interaction by means of smoothed particle hydrodynamics (SPH). Our code implements new equations of motion that allow for efficient use of non-equal mass particles and is capable of evolving contact binaries for thousands of orbits, if not indefinitely. We find that, in the majority of the cases considered, all three stars merge together. In addition, we compare our SPH calculations against those of the sticky-sphere approximation. If one is not concerned with mass loss, then the sticky sphere approach gives the correct qualitative outcome in approximately 75 per cent of the cases considered. Among those cases in which the sticky-sphere algorithm identifies only two particular stars to collide, the hydrodynamic calculations find the same qualitative outcome in about half of the instances. If the sticky-sphere approach determines that all three stars merge, then the hydrodynamic simulations invariably agree. However, in such three-star mergers, the hydrodynamic simulations reveal that: (i) mass lost as ejecta can be a considerable fraction of the total mass in the system (up to ∼25 per cent); (ii) due to asymmetric mass loss, the collision product can sometimes receive a kick velocity that exceeds 10 km s−1, large enough to allow the collision product to escape the core of the cluster; and (iii) the energy of the ejected matter can be large enough (up to ∼3 × 1050 erg) to remove or disturb the inter cluster gas appreciably.