On column density thresholds and the star formation rate

TL;DR

This study uses numerical simulations to investigate the existence of a column density threshold for star formation in molecular clouds, finding that star formation occurs only above a specific column density and correlates strongly with well-shielded gas mass.

Contribution

It demonstrates that star formation is linked to a column density threshold and clarifies the relationship between star formation rate and gas properties, emphasizing dust shielding effects.

Findings

Star formation occurs only when mean column density exceeds 10^{21} cm^{-2}.

Strong correlation between star formation rate and mass of cold, well-shielded gas.

Weaker, more time-dependent correlation between SFR and dense gas mass.

Abstract

We present the results of a numerical study designed to address the question of whether there is a column density threshold for star formation within molecular clouds. We have simulated a large number of different clouds, with volume and column densities spanning a wide range of different values, using a state-of-the-art model for the coupled chemical, thermal and dynamical evolution of the gas. We show that star formation is only possible in regions where the mean (area-averaged) column density exceeds $10^{21} \: {\rm cm^{-2}}$. Within the clouds, we also show that there is a good correlation between the mass of gas above a K-band extinction $A_{\rm K} = 0.8$ and the star formation rate (SFR), in agreement with recent observational work. Previously, this relationship has been explained in terms of a correlation between the SFR and the mass in dense gas. However, we find that this correlation is weaker and more time-dependent than that between the SFR and the column density. In support of previous studies, we argue that dust shielding is the key process: the true correlation is one between the SFR and the mass in cold, well-shielded gas, and the latter correlates better with the column density than the volume density.