Direct Numerical Simulations of Compressible Magnetohydrodynamical Turbulence

Summary

We report the results of three-dimensional, direct numerical simulations of compressible MHD turbulence relevant to the internal dynamics of molecular clouds. Models of both driven and decaying turbulence are considered. The decay rate of driven supersonic MHD turbulence is found to be large, of order of one eddy turnover time at the driving scale. Non-ideal MHD effects can increase this decay rate by a factor of about two. In models where the magnetic field is strong (strong enough that the velocity dispersion in the saturated state is less than the Alfven speed), the power spectrum of the turbulence is remarkably similar to the expectations of the theory of incompressible MHD turbulence.

Introduction

Numerical tools are likely to play an important role in the investigation of MHD turbulence in cold molecular clouds if for no other reason than because the observed linewidths are highly supersonic, and as of yet there does not exist a comprehensive analytic theory of compressible MHD turbulence. Our group (C. Gammie, E. Ostriker, and myself) has begun a project to study systematically the internal dynamics of magnetized, self-gravitating molecular clouds in two- and three-dimensions. Our motivations are two-fold: not only do we wish to understand the dynamics of compressible MHD turbulence as a well-defined physics problem, but also we would like to use the dynamical models as a basis with which to interpret the enormous collection of astronomical observations of molecular clouds that have been collected over the past several decades.