Abstract
This contribution re-examines the problem of ambipolar diffusion as a mechanism for the production and runaway evolution (collapse) of centrally condensed molecular cloud cores. The principal calculation applies in the geometric limit of a highly flattened core and allows for a semi-analytic treatment of the full problem. In this formulation, the ambipolar diffusion regime of evolution for negative times (t < 0) smoothly matches onto collapse solutions for positive times (t > 0). This 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. This work derives an analytic relationship between the dimensionless mass to flux ratio lambda_{0} equiv f_{0}(-1) of the central regions produced by runaway core condensation and the dimensionless rate of ambipolar diffusion epsilon; cores going into collapse typically have values of mass-to-flux ratio lambda_{0} ≈ 2. Next we show that ambipolar diffusion takes place more quickly in the presence of turbulent fluctuations, i.e., the effective value of the diffusion constant epsilon can be enhanced by turbulence. We also study self-similar collapse with the inclusion of nonzero initial inward velocities. Taken together, these findings show that the resulting theory provides a viable working paradigm for the formation of molecular cloud cores and their subsequent collapse to form stars and planetary systems.
