Do All Stars in the Solar Neighbourhood Form in Clusters?

TL;DR

This study analyzes the surface densities of young stellar objects in nearby star-forming regions, finding a smooth, lognormal distribution consistent with hierarchical cloud structures and indicating most stars form in less dense environments.

Contribution

It provides the first comprehensive analysis of YSO surface densities across multiple regions, challenging the idea of distinct star-formation modes and emphasizing the predominance of low-density formation.

Findings

YSO surface densities follow a lognormal distribution

Most stars form in low-density environments (<26%)

No evidence for multiple discrete star-formation modes

Abstract

We present a global study of low mass, young stellar object (YSO) surface densities (Sigma) in nearby (< 500 pc) star forming regions based on a comprehensive collection of Spitzer Space Telescope surveys. We show that the distribution of YSO surface densities is a smooth distribution, being adequately described by a lognormal function from a few to 10^3 YSOs per pc^2, with a peak at ~22 stars pc^-2. The observed lognormal Sigma is consistent with predictions of hierarchically structured star-formation at scales below 10 pc, arising from the molecular cloud structures. We do not find evidence for multiple discrete modes of star-formation (e.g. clustered and distributed). Comparing the observed Sigma distribution to previous Sigma threshold definitions of clusters show that they are arbitrary. We find that only a low fraction (< 26$) of stars are formed in dense environments where their formation/evolution (along with their circumstellar disks and/or planets) may be affected by the close proximity of their low-mass neighbours.