What Sets the Star Formation Rate of Molecular Clouds? The Density Distribution as a Fingerprint of Compression and Expansion Rates

TL;DR

This study uses 3D simulations to analyze how gas compression and expansion at different densities influence star formation rates, highlighting the role of turbulence, feedback, and gravity in molecular cloud dynamics.

Contribution

It reveals that the transition density in the gas density PDF marks the peak in mass flux and determines the star formation rate, emphasizing the importance of the density distribution as a diagnostic.

Findings

Mass flux peaks at the transition between lognormal and power-law PDF forms.

Gas dynamics at high densities are dominated by self-gravity.

Star formation rate correlates with the density where net gas flux equals star formation rate.

Abstract

We use a suite of 3D simulations of star-forming molecular clouds, with and without stellar feedback, magnetic fields, and driven turbulence, to study the compression and expansion rates of the gas as functions of density. We show that, around the mean density, supersonic turbulence promotes rough equilibrium between the amounts of compressing and expanding gas, consistent with continuous gas cycling between high and low density states. We find that the inclusion of protostellar jets produces rapidly expanding and compressing low-density gas. We find that the gas mass flux peaks at the transition between the lognormal and power-law forms of the density probability distribution function (PDF). This is consistent with the transition density tracking the post-shock density, which promotes an enhancement of mass at this density (i.e., shock compression and filament formation). At high densities, the gas dynamics are dominated by self-gravity: the compression rate in all of our runs matches the rate of the run with only gravity, suggesting that processes other than self-gravity have little effect at these densities. The net gas mass flux becomes constant at a density below the sink formation threshold, where it equals the star formation rate. The density at which the net gas mass flux equals the star formation rate is one order of magnitude lower than our sink threshold density, corresponds to the formation of the second power-law tail in the density PDF, and sets the overall star formation rates of these simulations.