Protostellar Jets and Outflows in low-mass star formation
  The driving mechanism of protostellar outflows and jets and their effects on
the star formation process obtained from recent theoretical and numerical
studies are described. Low-velocity outflows are driven by an outer region of
the circumstellar disk, while high-velocity jets are driven near an inner edge
of the disk. The disk angular momentum is effectively transferred by magnetic
effects in the outflow and jet driving regions where the magnetic field is well
coupled with neutral gas. On the other hand, in a high density gas region of
the disk (or intermediate region), the magnetic field dissipates and is
decoupled from neutral gas. Thus, in such a magnetically inactive region, no
outward flow appears and the disk angular momentum is not effectively
transferred by magnetic effects. Therefore, in the disk intermediate region,
the disk surface density continues to increase and gravitational instability
occurs and produce a non-axisymmetric (or spiral) structure. After spiral arms
sufficiently develop, the disk angular momentum is transferred by gravitational
torque and a large amount of the disk mass accretes onto the central protostar
from the circumstellar disk. The episodic accretion induces time-variable
high-velocity jets. The jets do not significantly contribute to a dynamical
evolution of the protostar and circumstellar disk, while the low-velocity
outflows can eject a large fraction of the infalling gas and determine the
final stellar mass.
