A Rotating Protostellar Jet Launched from the Innermost Disk of HH 212
  The central problem in forming a star is the angular momentum in the
circumstellar disk which prevents material from falling into the central
stellar core. An attractive solution to the "angular momentum problem" appears
to be the ubiquitous (low-velocity and poorly-collimated) molecular outflows
and (high-velocity and highly-collimated) protostellar jets accompanying the
earliest phase of star formation that remove angular momentum at a range of
disk radii. Previous observations suggested that outflowing material carries
away the excess angular momentum via magneto-centrifugally driven winds from
the surfaces of circumstellar disks down to ~ 10 AU scales, allowing the
material in the outer disk to transport to the inner disk. Here we show that
highly collimated protostellar jets remove the residual angular momenta at the
~ 0.05 AU scale, enabling the material in the innermost region of the disk to
accrete toward the central protostar. This is supported by the rotation of the
jet measured down to ~ 10 AU from the protostar in the HH 212 protostellar
system. The measurement implies a jet launching radius of ~
0.05_{-0.02}^{+0.05} AU on the disk, based on the magneto-centrifugal theory of
jet production, which connects the properties of the jet measured at large
distances to those at its base through energy and angular momentum
conservation.
