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

Understand

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).
• We find that, in the majority of the cases considered, all three stars merge together, and in such three star mergers, the hydrodynamic simulations reveal that: (1) mass lost as ejecta can be a considerable fraction of the total mass in the system (up to $\sim25$%); (2) due to asymmetric mass loss, the collision product can sometimes receive a kick velocity that exceeds 10 km/s, large enough to allow the collision product to escape the core of the cluster; and (3) the energy of the ejected matter can be large enough (up to $\sim 3\times 10^{50}$ erg) to remove or disturb the inter cluster gas appreciably.