MHD Turbulence in Star-Forming Regions and the Interstellar Medium

Abstract

MHD turbulence plays a central role in the physics of star-forming molecular clouds and the interstellar medium. MHD turbulence in molecular clouds must be driven to account for the observed supersonic motions in the clouds, as even strongly magnetized turbulence decays quickly. Driven MHD turbulence can globally support gravitationally unstable regions, but local collapse inevitably occurs. Differences in the strength of driving and the gas density may explain the very different rates of star formation observed in different galaxies. Two types of comparisons to observations are reviewed. First, the use of wavelet transform methods suggests that the driving comes from scales larger than observed molecular clouds. Second, comparison of simulated spectral cubes from models to real observations suggests that Larson’s mass-size relationship is an observational artifact. The driving mechanism for the turbulence is likely a combination of field supernovae in star-forming sections of galactic disks, and magnetorotational instabilities in outer disks and lowsurface brightness galaxies. Supernova-driven turbulence has a broad range of pressures with a roughly log-normal distribution. High-pressure, cold regions can be formed even in the absence of selfgravity.