Do all stars in the solar neighbourhood form in clusters? A cautionary note on the use of the distribution of surface densities

TL;DR

The paper argues that the observed surface density distribution of young stars can be explained by all stars forming in clusters that expand over time, challenging the idea that stars predominantly form in hierarchical, dispersed environments.

Contribution

It demonstrates that the surface density distribution is consistent with all stars forming in bound clusters that expand, questioning previous interpretations of star formation modes.

Findings

Surface density distribution can result from cluster expansion.

Cluster properties explain the observed distribution.

Degeneracy complicates using surface density as a formation environment indicator.

Abstract

Bressert et al. recently showed that the surface density distribution of low-mass, young stellar objects (YSOs) in the solar neighbourhood is approximately log-normal. The authors conclude that the star formation process is hierarchical and that only a small fraction of stars form in dense star clusters. Here, we show that the peak and the width of the density distribution are also what follow if all stars form in bound clusters which are not significantly affected by the presence of gas and expand by two-body relaxation. The peak of the surface density distribution is simply obtained from the typical ages (few Myr) and cluster membership number (few hundred) typifying nearby star-forming regions. This result depends weakly on initial cluster sizes, provided that they are sufficiently dense (initial half mass radius of <0.3 pc) for dynamical evolution to be important at an age of a few Myr. We conclude that the degeneracy of the YSO surface density distribution complicates its use as a diagnostic of the stellar formation environment.