  Dynamical collapses of magnetized molecular cloud cores are studied with
magnetohydrodynamical simulations from the run-away collapse phase to the
accretion phase. In the run-away collapse phase, a disk threaded by magnetic
field lines is contracting due to its self-gravity and its evolution is well
expressed by a self-similar solution. The central density increases greatly in
a finite time scale and reaches a density at which an opaque core is formed at
the center. After that, matter accretes to the newly formed core (accretion
phase). In this stage, a rotationally supported disk is formed in a cloud core
without magnetic fields. In contrast, the disk continues to contract in the
magnetized cloud core, since the magnetic fields transfer angular momentum from
the disk. Its rotation motion winds up the threading magnetic field lines.
Eventually, strong toroidal magnetic fields are formed and begin to drive the
outflow, even if there is no toroidal field component initially. Bipolar
molecular outflows observed in protostar candidates are naturally explained by
this model.
