Supernova Driving. II. Compressive Ratio in Molecular-Cloud Turbulence

TL;DR

This study analyzes molecular cloud turbulence driven by supernovae, revealing a broad distribution of compressive ratios and their relation to turbulence statistics, with implications for star formation models.

Contribution

It provides the first analysis of compressive ratios in realistic supernova-driven turbulence simulations of molecular clouds, contrasting with idealized force-driven models.

Findings

SN-driven turbulence has a mean compressive ratio of ~0.3

Compressibility is unaffected by gravity or cloud rotation

Gas density PDFs are lognormal with high-density tails when gravity is included

Abstract

The compressibility of molecular cloud (MC) turbulence plays a crucial role in star formation models, because it controls the amplitude and distribution of density fluctuations. The relation between the compressive ratio (the ratio of powers in compressive and solenoidal motions) and the statistics of turbulence has been previously studied systematically only in idealized simulations with random external forces. In this work, we analyze a simulation of large-scale turbulence (250 pc) driven by supernova (SN) explosions that has been shown to yield realistic MC properties. We demonstrate that SN driving results in MC turbulence with a broad lognormal distribution of the compressive ratio, with a mean value $\approx 0.3$, lower than the equilibrium value of $\approx 0.5$ found in the inertial range of isothermal simulations with random solenoidal driving. We also find that the compressibility of the turbulence is not noticeably affected by gravity, nor are the mean cloud radial (expansion or contraction) and solid-body rotation velocities. Furthermore, the clouds follow a general relation between the rms density and the rms Mach number similar to that of supersonic isothermal turbulence, though with a large scatter, and their average gas density PDF is described well by a lognormal distribution, with the addition of a high-density power-law tail when self-gravity is included.