Compressible MHD Turbulence: Implications for Molecular Cloud and Star Formation

TL;DR

This review synthesizes recent numerical simulation results on MHD turbulence in the interstellar medium and molecular clouds, highlighting its role in cloud formation, evolution, and star formation, with implications for physical properties and structure scaling.

Contribution

It provides a comprehensive overview of recent advances in modeling MHD turbulence in the ISM, including the effects of magnetic fields, gravity, and turbulence on cloud structures and star formation.

Findings

Velocity-size scaling matches observations

No universal density-size scaling law

Turbulent decay time shorter than free-fall time

Abstract

We review recent results from numerical simulations and related models of MHD turbulence in the interstellar medium (ISM) and in molecular clouds. We discuss the implications of turbulence for the processes of cloud formation and evolution, and the determination of clouds' physical properties. Numerical simulations of the turbulent ISM to date have included magnetic fields, self-gravity, parameterized heating and cooling, modeled star formation and other turbulent inputs. The structures which form reproduce well observed velocity-size scaling properties, while predicting the non-existence of a general density-size scaling law. Criteria for the formation of gravitationally-bound structures by turbulent compression are summarized. For flows with equations of state $P\propto \rho^\gamma$, the statistics of the density field depend on the exponent $\gamma$. Numerical simulations of both forced and decaying MHD compressible turbulence have shown that the decay rate is comparable to the non-magnetic case. For virialized clouds, the turbulent decay time is shorter than the gravitational free-fall time, so wholesale cloud collapse is only prevented by ongoing turbulent inputs and/or a strong mean magnetic field. Finally, perspectives for future work in this field are briefly discussed.