Collisional formation of very massive stars in dense clusters

TL;DR

This study models how dense star clusters can produce very massive stars through collisions and accretion, revealing rapid runaway growth and implications for cluster evolution.

Contribution

It extends n-body simulations to include accretion, demonstrating conditions for stellar collisions and runaway growth in dense clusters, a novel approach to understanding massive star formation.

Findings

Runaway growth occurs within 1 Myr after accretion stops.

Collisions are significant only in very dense, large-n clusters.

Accretion-driven shrinkage helps offset gas expulsion effects.

Abstract

We investigate the contraction of accreting protoclusters using an extension of n-body techniques that incorporates the accretional growth of stars from the gaseous reservoir in which they are embedded. Following on from Monte Carlo studies by Davis et al., we target our experiments toward populous clusters likely to experience collisions as a result of accretion-driven contraction. We verify that in less extreme star forming environments, similar to Orion, the stellar density is low enough that collisions are unimportant, but that conditions suitable for stellar collisions are much more easily satisfied in large-n clusters, i.e. n ~ 30,000 (we argue, however, that the density of the Arches cluster is insufficient for us to expect stellar collisions to have occurred in the cluster's prior evolution). We find that the character of the collision process is not such that it is a route toward smoothly filling the top end of the mass spectrum. Instead, runaway growth of one or two extreme objects can occur within less than 1 Myr after accretion is shut off, resulting in a few objects with masses several times the maximum reached by accretion. The rapid formation of these objects is due to not just the post-formation dynamical evolution of the clusters, but an interplay of dynamics and the accretional growth of the stars. We find that accretion-driven cluster shrinkage results in a distribution of gas and stars that offsets the disruptive effect of gas expulsion, and we propose that the process can lead to massive binaries and early mass segregation in star clusters.