The evolution of the mass--radius relation of expanding very young star clusters

TL;DR

This study uses N-body simulations and observational data to analyze the early expansion of young star clusters, revealing how their mass-radius relation evolves and suggesting they are often in an expanding state shortly after formation.

Contribution

The paper demonstrates that the observed size evolution of young clusters aligns with models of expansion from initial conditions, clarifying the initial mass-radius relation of embedded clusters.

Findings

Observed young clusters are likely in an expansion phase.

The mass-radius relation of Pfalzner et al. reflects post-expansion evolution.

Embedded clusters in ATLASGAL are probably already expanding.

Abstract

The initial mass--radius relation of embedded star clusters is an essentially important boundary condition for understanding the evolution of embedded clusters in which stars form to their release into the galactic field via an open star cluster phase. The initial mass--radius relation of embedded clusters deduced by Marks \& Kroupa is significantly different from the relation suggested by Pfalzner et al. Here, direct N-body simulations are used to model the early expansion of embedded clusters after the expulsion of their residual gas. The observational data of radii of up to a few~Myr old clusters collated from various sources are found to be well fit by the N-body models, implying these observed very young clusters to most likely be in an expanding state. We show that the mass-radius relation of Pfalzner et al. reflects the expansion of embedded clusters following the initial mass-radius relation of Marks \& Kroupa. We also suggest that even the embedded clusters in ATLASGAL clumps with HII regions are probably already in expansion. All here collected clusters from different observations show a mass-size relation with a similar slope, which may indicate that all clusters were born with a profile resembling that of the Plummer phase-space distribution function.