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.