Connection between dense gas mass fraction, turbulence driving, and star formation efficiency of molecular clouds

TL;DR

This study investigates how turbulence driving modes influence the dense gas mass fraction in molecular clouds and its relation to star formation efficiency, using simulations and comparing with observations.

Contribution

It demonstrates that the turbulence driving mode significantly affects the dense gas fraction and star formation efficiency in molecular clouds, providing a link between turbulence properties and star formation.

Findings

Exponential DGMFs are predicted over certain column densities.

DGMF slopes correlate with turbulence driving type and star formation efficiency.

Simulations with less compressive turbulence match observed DGMFs of nearby clouds.

Abstract

We examine the physical parameters that affect the accumulation of gas in molecular clouds to high column densities where the formation of stars takes place. In particular, we analyze the dense gas mass fraction (DGMF) in a set of self-gravitating, isothermal, magnetohydrodynamic turbulence simulations including sink particles to model star formation. We find that the simulations predict close to exponential DGMFs over the column density range N(H2) = 3-25 x 10^{21} cm^{-2} that can be easily probed via, e.g., dust extinction measurements. The exponential slopes correlate with the type of turbulence driving and also with the star formation efficiency. They are almost uncorrelated with the sonic Mach number and magnetic-field strength. The slopes at early stages of cloud evolution are steeper than at the later stages. A comparison of these predictions with observations shows that only simulations with relatively non-compressive driving (b ~< 0.4) agree with the DGMFs of nearby molecular clouds. Massive infrared dark clouds can show DGMFs that are in agreement with more compressive driving. The DGMFs of molecular clouds can be significantly affected by how compressive the turbulence is on average. Variations in the level of compression can cause scatter to the DGMF slopes, and some variation is indeed necessary to explain the spread of the observed DGMF slopes. The observed DGMF slopes can also be affected by the clouds' star formation activities and statistical cloud-to-cloud variations.