Dynamical Collapse of Nonrotating Magnetic Molecular Cloud Cores: Evolution through Point‐Mass Formation

We present a numerical simulation of the dynamical collapse of a nonrotatingmagnetic molecular cloud core and follow the core's evolution through theformation of a central point mass and its subsequent growth to a 1 solar-massprotostar. The epoch of point-mass formation (PMF) is investigated by a self-consistent extension of previously presented models of core formation andcontraction in axisymmetric, self-gravitating, isothermal, magneticallysupported interstellar molecular clouds. Prior to PMF, the core is dynamicallycontracting and is not well approximated by a quasistatic equilibrium model.Ambipolar diffusion, which plays a key role in the early evolution of the core,is unimportant during the dynamical pre-PMF collapse phase. However, theappearance of a central mass, through its effect on the gravitational field inthe inner core regions, leads to a "revitalization" of ambipolar diffusion inthe weakly ionized gas surrounding the central protostar. This process is soefficient that it leads to a decoupling of the field from the matter andresults in an outward propagating hydromagnetic C-type shock. The existence ofan ambipolar diffusion-mediated shock was predicted by Li & McKee (1996), andwe find that the basic shock structure given by their analytical model is wellreproduced by our more accurate numerical results. Our calculation alsodemonstrates that ambipolar diffusion, rather than Ohmic diffusivity operatingin the innermost core region, is the main field decoupling mechanismresponsible for driving the shock after PMF.Comment: 59 pages, 10 figures, AASTeX4.0 accepted for publication in The Astrophysical Journa