  We present a numerical simulation of the dynamical collapse of a nonrotating
magnetic molecular cloud core and follow the core's evolution through the
formation of a central point mass and its subsequent growth to a 1 solar-mass
protostar. The epoch of point-mass formation (PMF) is investigated by a self-
consistent extension of previously presented models of core formation and
contraction in axisymmetric, self-gravitating, isothermal, magnetically
supported interstellar molecular clouds. Prior to PMF, the core is dynamically
contracting 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, the
appearance of a central mass, through its effect on the gravitational field in
the inner core regions, leads to a "revitalization" of ambipolar diffusion in
the weakly ionized gas surrounding the central protostar. This process is so
efficient that it leads to a decoupling of the field from the matter and
results in an outward propagating hydromagnetic C-type shock. The existence of
an ambipolar diffusion-mediated shock was predicted by Li & McKee (1996), and
we find that the basic shock structure given by their analytical model is well
reproduced by our more accurate numerical results. Our calculation also
demonstrates that ambipolar diffusion, rather than Ohmic diffusivity operating
in the innermost core region, is the main field decoupling mechanism
responsible for driving the shock after PMF.
