Magnetic Fields in Molecular Clouds: Observations Confront Theory

This paper presents a summary of all 27 available sensitive Zeeman measurements of magnetic field strengths in molecular clouds together with other relevant physical parameters. From these data input parameters to magnetic star formation theory are calculated, and predictions of theory are compared with observations. Results for this cloud sample are the following: (1) Internal motions are supersonic but approximately equal to the Alfvén speed, which suggests that supersonic motions are likely MHD waves. (2) The ratio of thermal to magnetic pressures β p ≈ 0.04, implying that magnetic fields are important in the physics of molecular clouds. (3) The mass-to-magnetic flux ratio is about twice critical, which suggests but does not require that static magnetic fields alone are insufficient to support clouds against gravity. (4) Kinetic and magnetic energies are approximately equal, which suggests that static magnetic fields and MHD waves are roughly equally important in cloud energetics. (5) Magnetic field strengths scale with gas densities as | B | ∝ ρ κ with κ ≈ 0.47; this agrees with the prediction of ambipolar diffusion driven star formation, but this scaling may also be predicted simply by Alfvénic motions. The measurements of magnetic field strengths in molecular clouds make it clear that magnetic fields are a crucial component of the physics governing cloud evolution and star formation.