Universality of young cluster sequences

TL;DR

This paper reveals that young star clusters follow two main evolutionary sequences in density and size, allowing cluster age estimation based on size, challenging the idea of a wide range of initial cluster densities.

Contribution

It demonstrates that cluster densities and sizes evolve along two main sequences, providing a new method to determine cluster ages from their size.

Findings

Star clusters evolve along two main sequences in density-radius space.

Cluster size can be used as an indicator of age.

Cluster formation occurs under tight density constraints.

Abstract

Most stars do not form in isolation but as part of a cluster comprising anywhere between a few dozen to several million stars with stellar densities ranging from 0.01 to several 10$^5$ \Msun pc$^{-3}$. The majority of these clusters dissolve within 20 Myr. The general assumption is that clusters are born more or less over this entire density range. A new analysis of cluster observations is presented. It demonstrates that, in fact, clustered star formation works under surprisingly tight constraints with respect to cluster size and density. The observed multitude of cluster densities simply results from snapshots of two sequences evolving in time along pre-defined tracks in the density-radius plane. This implies that the cluster size can actually be used to determine its age.