Ambipolar Diffusion in Molecular Cloud Cores and the Gravomagneto Catastrophe

This paper re-examines the problem of ambipolar diffusion as a mechanism for the production and runaway evolution of centrally condensed molecular cloud cores, a process that has been termed the gravomagneto catastrophe. Our calculation applies in the geometric limit of a highly flattened core and allows for a semi-analytic treatment of the full problem, although physical fixes are required to resolve a poor representation of the central region. A noteworthy feature of the overall formulation is that the solutions for the ambipolar diffusion portion of the evolution for negative times (t < 0) match smoothly onto the collapse solutions for positive times (t > 0). The treatment shows that the resulting cores display non-zero, but sub-magnetosonic, inward velocities at the end of the diffusion epoch, in agreement with current observations. Another important result is the derivation of an analytic relationship between the dimensionless mass to flux ratio λ0 ≡ f 0 of the central regions produced by runaway core condensation and the dimensionless measure of the rate of ambipolar diffusion ǫ. In conjunction with previous work showing that ambipolar diffusion takes place more quickly in the presence of turbulent fluctuations, i.e., that the effective value of ǫ can be enhanced by turbulence, the resultant theory provides a viable working hypothesis for the formation of isolated molecular-cloud cores and their subsequent collapse to form stars and planetary systems. Subject headings: MHD — methods: analytical — stars: formation