The early dynamical evolution of cool, clumpy star clusters

TL;DR

This study uses N-body simulations to explore how cool, clumpy star clusters evolve dynamically, revealing rapid mass segregation, formation of multiple systems, ejections of massive stars, and potential for core collapse that can disrupt clusters.

Contribution

It provides new insights into the early dynamical processes of star clusters, especially the rapid evolution and formation of complex structures without gas expulsion.

Findings

Clusters undergo rapid mass segregation.

Massive stars are often ejected at high velocities.

Core collapse can lead to cluster disruption.

Abstract

Observations and theory both suggest that star clusters form sub-virial (cool) with highly sub-structured distributions. We perform a large ensemble of N-body simulations of moderate-sized (N=1000) cool, fractal clusters to investigate their early dynamical evolution. We find that cool, clumpy clusters dynamically mass segregate on a short timescale, that Trapezium-like massive higher-order multiples are commonly formed, and that massive stars are often ejected from clusters with velocities > 10 km/s (c.f. the average escape velocity of 2.5 km/s). The properties of clusters also change rapidly on very short timescales. Young clusters may also undergo core collapse events, in which a dense core containing massive stars is hardened due to energy losses to a halo of lower-mass stars. Such events can blow young clusters apart with no need for gas expulsion. The warmer and less substructured a cluster is initially, the less extreme its evolution.