Observations of Magnetic Fields in Dense Interstellar Clouds: Implications for MHD Turbulence and Cloud Evolution

Summary

We discuss the role that magnetic fields may play in the dynamics and evolution of dense interstellar clouds. We review techniques for observation of magnetic field strengths in molecular clouds and results of observations of the Zeeman effect. Observed field strengths range from 0.03 to 3 milligauss and the gas densities range over log(n) ≈ 4–7. These data are used to compute the mass to magnetic flux ratios and the ratios of the observed internal speeds to the Alfven speeds, in order to asses the importance of static magnetic fields in cloud support and the extent to which internal motions are Alfvenic or sub-Alfvenic.

Introduction

Over the last several decades it has become clear that the dynamics and evolution of star-forming interstellar clouds are difficult to explain without magnetic effects. A principal problem involves support of dense clouds against their own gravity. In general, such clouds are observed to be in approximate virial equilibrium between gravity and internal motions. Seemingly, therefore, they should be stable against collapse. However, observed line widths are almost invariably much greater than the sound speed. Therefore the internal motions that support the clouds are highly supersonic, and simple estimates indicate that shock-induced dissipation of mechanical energy should occur on about the free-fall time. In such a case, non-magnetic turbulence offers no effective support for the clouds (unless, of course, it can somehow be continuously regenerated).