25 AU Angular Resolution Observations of HH 211 with ALMA : Jet
  Properties and Shock Structures in SiO, CO, and SO
  HH 211 is a highly collimated jet with a chain of knots and a wiggle
structure on both sides of a young Class 0 protostar. We used two epochs of
Atacama Large Millimeter/submillimeter Array (ALMA) data to study its inner jet
in the CO(J=3-2), SiO(J=8-7), and SO(N_J=8_9-7_8) lines at $\sim$25 AU
resolution. With these ALMA and previous 2008 Submillimeter Array (SMA) data,
the proper motion of 8 knots within $\sim$250 AU of the central source is found
to be $\sim$0.068" per year ($\sim$102 km/s), consistent with previous
measurements in the outer jet. At $\sim$4 times higher resolution, the
reflection-symmetric wiggle can be still fitted by a previously proposed
orbiting jet source model. Previously detected continuous structures in the
inner jet are now resolved, containing at least 5 sub-knots. These sub-knots
are interpreted in terms of a variation in the ejection velocity of the jet
with a period of $\sim$4.5 years, shorter than that of the outer knots. In
addition, backward and forward shocks are resolved in a fully-formed knot, BK3,
and signatures of internal working surface and sideways ejection are identified
in Position-Velocity diagrams. In this knot, low-density SO and CO layers are
surrounded by a high-density SiO layer.
