Signatures of star cluster formation by cold collapse

TL;DR

This study uses simulations to identify observable signatures of star cluster formation via cold collapse, highlighting the rapid evolution of initial conditions and the importance of early or large-scale observations.

Contribution

It provides the first detailed simulation-based analysis of morphological and kinematic signatures of sub-virial collapse in young star clusters.

Findings

Initial signatures are quickly erased during collapse.

Early or large-scale observations are crucial for detecting formation signatures.

Current conditions may be misleading for studying star formation rates.

Abstract

Sub-virial gravitational collapse is one mechanism by which star clusters may form. Here we investigate whether this mechanism can be inferred from observations of young clusters. To address this question, we have computed SPH simulations of the initial formation and evolution of a dynamically young star cluster through cold (sub-virial) collapse, starting with an ellipsoidal, turbulently seeded distribution of gas, and forming sink particles representing (proto)stars. While the initial density distributions of the clouds do not have large initial mass concentrations, gravitational focusing due to the global morphology leads to cluster formation. We use the resulting structures to extract observable morphological and kinematic signatures for the case of sub-virial collapse. We find that the signatures of the initial conditions can be erased rapidly as the gas and stars collapse, suggesting that kinematic observations need to be made either early in cluster formation and/or at larger scales, away from the growing cluster core. Our results emphasize that a dynamically young system is inherently evolving on short timescales, so that it can be highly misleading to use current-epoch conditions to study aspects such as star formation rates as a function of local density. Our simulations serve as a starting point for further studies of collapse including other factors such as magnetic fields and stellar feedback.