Modelling Star Cluster Formation: Mergers

TL;DR

This study uses detailed simulations to explore how subclusters in giant molecular clouds collide and merge, revealing complex dynamics and challenging simplified models of star cluster formation.

Contribution

It introduces a more detailed simulation approach for subcluster mergers, moving beyond simplified sink particle models to better understand cluster formation.

Findings

High-velocity collisions prevent monolithic cluster formation within 3 Myr.

Clusters often do not reach equilibrium and continue expanding.

Standard density profiles do not fit the resulting cluster structures.

Abstract

Star cluster formation in giant molecular clouds involves the local collapse of the cloud into small gas-rich subclusters, which can then subsequently collide and merge to build up the final star cluster(s). In this paper, we simulate collisions between these subclusters, using coupled smooth particle hydrodynamics for the gas and N-body dynamics for the stars. We are guided by previous radiation hydrodynamics simulations of molecular cloud collapse which provide the global properties of the colliding clusters, such as their stellar and gas masses, and their initial positions and velocities. The subclusters in the original simulation were treated as sink particles which immediately merged into a single entity after the collision. We show that the more detailed treatment provides a more complex picture. At collisional velocities above ~ 10 km/s, the stellar components of the cluster do not form a monolithic cluster within 3 Myr, although the gas may do so. At lower velocities, the clusters do eventually merge but over timescales that may be longer than the time for a subsequent collision. The structure of the resultant cluster is not well-fit by any standard density distribution, and the clusters are not in equilibrium but continue to expand over our simulation time. We conclude that the simple sink particle treatment of subcluster mergers in large-scale giant molecular cloud simulations provides an upper limit on the final cluster properties.