The growth of massive stars via stellar collisions in ensemble star clusters

TL;DR

This study uses N-body simulations to explore how the timing of sub-cluster assembly affects the formation of massive stars through stellar collisions, revealing that early assembly enhances collision efficiency and star mass growth.

Contribution

It demonstrates that the timing of sub-cluster assembly relative to core collapse significantly influences the efficiency of stellar collisions and the resulting star masses in cluster evolution.

Findings

Early assembly leads to more efficient stellar collisions.

Sub-clusters assembling after individual core collapse produce fewer collision products.

Remnant clusters from early assembly have higher central densities and more massive stars.

Abstract

Recent simulations and observations suggest that star clusters form via the assembling of smaller sub-clusters. Because of their short relaxation time, sub-clusters experience core collapse much earlier than virialized solo-clusters, which have similar properties of the merger remnant of the assembling clusters. As a consequence it seems that the assembling clusters result in efficient multiple collisions of stars in the cluster core. We performed a series of $N$-body simulations of ensemble and solitary clusters including stellar collisions and found that the efficiency of multiple collisions between stars are suppressed if sub-clusters assemble after they experience core collapse individually. In this case, sub-clusters form their own multiple collision stars which experienced a few collisions, but they fail to collide with each other after their host sub-clusters assemble. The multiple collision stars scatter each other and escape, and furthermore the central density of the remnant clusters had already been depleted for the stars to experience more collisions. On the other hand, if sub-clusters assemble before they experience core collapse, the multiple collisions of stars proceed efficiently in the remnant cluster, and the collision products are more massive than virialized solo-clusters and comparable in mass to cold solo-clusters.