The early dynamical evolution of cool, clumpy star clusters

Observations and theory both suggest that star clusters form subvirial (cool) with highly substructured 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 time-scale, that Trapezium-like massive higher order multiples are commonly formed, and that massive stars are often ejected from clusters with velocities >10 km s−1 (cf. the average escape velocity of 2.5 km s−1). The properties of clusters also change rapidly on very short time-scales. 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.