Star formation in the presence of magnetic fields: Molecular cloud core formation and dynamical contraction due to ambipolar diffusion; a simulation with axial symmetry

Interstellar molecular clouds are generally believed to be the sites of active star formation in our galaxy. Observed densities, temperatures, and magnetic field strengths imply that well-ordered magnetic fields dominate thermal pressure in supporting these objects against self-gravity. The magnetic force is transferred to the neutrals through collisions with ions. Because the degree of ionization drops as the density increases, redistribution of the mass-to-flux ratio in a cloud's central flux tubes (ambipolar diffusion) takes place at an ever-increasing rate, leading to the formation of high-density cores, typically after 8 million years of quasi-static contraction. Eventually, the central mass-to-flux ratio exceeds the critical value for collapse, and the core begins to contract rapidly.We solve numerically the full non-linear, non-ideal MHD equations by using a new, non-orthogonal, fully adaptive grid describing the evolution of a set of non-rotating, axially symmetric model clouds. We follow the evolution from a central density $n\sb{\rm c}$ of 300 cm$\sp{-3}$ to 10$\sp9$ cm$\sp{-3}$, at which point the calculation is stopped because the relation for the ionization fraction and the assumption of isothermality begin to break down. The central field strength $B\sb{\rm c}$ increases from 16-120 microgauss to 1-5 milligauss for the ranges of initial values of the dimensionless parameters studied. Even when the final degree of ionization is a factor of 80 smaller than in the typical case, the field lines are pulled inward in the core during its rapid collapse. In the relation $B\sb{\rm c}\ \propto\ n\sb{\rm c}\sp\kappa$, we find 0.42 $\leq\ \kappa\ \leq$ 0.48.The central mass-to-flux ratio increases by a factor of from 10 (for canonical parameter values) up to 60 (for initial states that are a factor of 10 magnetically subcritical, and also for models that have relatively few ions, where most of the increase occurs during dynamical collapse). The extended envelope retains its magnetic support long after the core contracts.U of I OnlyETDs are only available to UIUC Users without author permissio