The Evolution of Self-Gravitating, Magnetized, Turbulent Clouds: Numerical Experiments

Summary

Interstellar giant molecular clouds and dark clouds are observed to have comparable kinetic and gravitational energies, and low enough temperatures that their internal turbulent velocity amplitudes are highly supersonic. It has been believed for some time that the presence of magnetic fields can have important consequences for the properties of turbulence in these clouds, and for cloud's gravitational stability. In this paper, I outline how the physical parameters of clouds can be translated to dimensionless ratios (the Mach number, the Jeans number, and the plasma β), describe a series of numerical experiments underway to evaluate how the character of the turbulence depends on these parameters, and present selections from our results to date.

Introduction

The turbulence in Galactic molecular clouds has a rather different character from the other forms of interstellar turbulence discussed at this meeting. Strong molecular cooling brings the ambient temperatures to the range T = 10 – 30 K, which renders turbulence with velocities of a few km s⁻¹ not just nonlinear, but hypersonic. Most observational evidence on magnetic field strengths suggest that Alfvén speeds are of same order than the turbulent speeds or a few times larger, and in any case unlikely to be smaller than the sound speed. Thus, the turbulence in molecular clouds is highly compressible, and strongly magnetic. In addition, although high-latitude unbound molecular clouds exist, most of the molecular material in the Galaxy resides in much more massive, self-gravitating, giant molecular clouds and cloud complexes (GMCs).