Accretion-driven core collapse and the collisional formation of massive stars

TL;DR

This paper investigates how gas accretion can cause core collapse in star clusters, increasing stellar collision likelihood, especially in massive clusters, potentially leading to intermediate mass black hole formation.

Contribution

It introduces a model linking core accretion, density increase, and collision probability, highlighting conditions favoring stellar collisions in massive clusters.

Findings

Collision likelihood scales with cluster core size and velocity.

Collisions are rare in low-mass clusters like Orion.

Massive clusters have a higher probability of stellar collisions.

Abstract

We consider the conditions required for a cluster core to shrink, by adiabatic accretion of gas from the surrounding cluster, to densities such that stellar collisions are a likely outcome. We show that the maximum densities attained, and hence the viability of collisions, depends on a competition between core shrinkage (driven by accretion) and core puffing up (driven by relaxation effects). The expected number of collisions scales as $N_{core}^{5/3} \tilde v^2$ where $N_{core}$ is the number of stars in the cluster core and $\tilde v$ is the free fall velocity of the parent cluster (gas reservoir). Thus whereas collisions are very unlikely in a relatively low mass, low internal velocity system such as the Orion Nebula Cluster, they become considerably more important at the mass and velocity scale characteristic of globular clusters. Thus stellar collisions in response to accretion induced core shrinkage remains a viable prospect in more massive clusters, and may contribute to the production of intermediate mass black holes in these systems.