Magnetic Effects in Clouds

Abstract

A realistic study of the structure and evolution of an interstellar gas cloud must take cognisance of the flux from the galactic magnetic field threading the cloud. If the non-dimensional mass-to-flux ratio is below a critical value, the forces exerted by the locally distorted field can balance gravity in the two trans-field dimensions, while Alfvenic turbulent motions yield support along the field. A super-critical cloud, collapsing with its flux virtually frozen in, may fragment into sub-condensations following spontaneous flattening along the field. Within a sub-critical molecular cloud, the very low degree of ionisation allows the magnetic forces to redistribute flux through the cloud, so that locally denser regions may become super-critical and condense out of the cloud. The Maxwell stresses also transport angular momentum efficiently from a slowly contracting condensation to the surroundings. If flux leakage remains slow throughout all the pre-opaque phases, the magnetic forces and the associated turbulent motions may shift the ultimate mass spectrum towards the high mass end. Most of the remnant flux may be lost by magnetic buoyancy during the pre-main sequence epoch, so possibly supplying a power source for the T Tauri phenomenon.