Star Formation in Magnetic Interstellar Clouds: II. Basic Theory

Abstract

Magnetic braking and ambipolar diffusion are unavoidable physical processes in interstellar clouds and play a key, perhaps even crucial, role in star formation. We summarize those calculations on magnetic braking and ambipolar diffusion which led to the proposal of the scenario for star formation described in the accompanying paper. This is not a review of the literature. New insights into the underlying physics of the two processes are provided, and further contact with observations is made. The mathematically complicated solutions for the resolution of the angular momentum problem by magnetic braking are elucidated through exact analogies with transverse waves on strings. Reduction of the magnetic flux theading a cloud’s core by 2 to 4 orders of magnitude due to ambipolar diffusion is demonstrated through detailed, one-dimensional collapse calculations and explained in physical terms. However, it is by no means a foregone conclusion that collapse calculations in higher dimensions will confirm this result; we explain why. The first, recent quantification of the effect of ambipolar diffusion on magnetic braking is described; application to molecular cloud cores yields surprising results, which are understood by analogy with electrical resistors connected in series and in parallel. Key problems still remaining unsolved are emphasized.