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

TL;DR

This study uses hydrodynamic simulations to analyze three-body stellar interactions in dense clusters, revealing significant mass loss, potential ejection velocities, and energetic ejecta that impact cluster evolution.

Contribution

It provides detailed hydrodynamic modeling of three-star encounters, highlighting the mass loss and dynamical effects during stellar collisions in clusters.

Findings

Mass loss as ejecta can reach up to 25% of total mass.

Collision products can receive kicks exceeding 10 km/s.

Ejected matter can carry energies up to 3×10^{50} erg.

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). 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.