Jet Motion, Internal Working Surfaces, and Nested Shells in the
  Protostellar System HH 212
  HH 212 is a nearby (400 pc) highly collimated protostellar jet powered by a
Class 0 source in Orion. We have mapped the inner 80" (~ 0.16 pc) of the jet in
SiO (J=8-7) and CO (J=3-2) simultaneously at ~ 0.5 resolution with the Atacama
Millimeter/Submillimeter Array at unprecedented sensitivity. The jet consists
of a chain of knots, bow shocks, and sinuous structures in between. As compared
to that seen in our previous observations with the Submillimeter Array, it
appears to be more continuous, especially in the northern part. Some of the
knots are now seen associated with small bow shocks, with their bow wings
curving back to the jet axis, as seen in pulsed jet simulations. Two of them
are reasonably resolved, showing kinematics consistent with sideways ejection,
possibly tracing the internal working surfaces formed by a temporal variation
in the jet velocity. In addition, nested shells are seen in CO around the jet
axis connecting to the knots and bow shocks, driven by them. The proper motion
of the jet is estimated to be ~ 115+-50 km/s, comparing to our previous
observations. The jet has a small semi-periodical wiggle, with a period of ~ 93
yrs. The amplitude of the wiggle first increases with the distance from the
central source and then stays roughly constant. One possible origin of the
wiggle could be the kink instability in a magnetized jet.
