25 AU Angular Resolution Observations of HH 211 with ALMA : Jet Properties and Shock Structures in SiO, CO, and SO
Abstract
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(NJ=89-78) lines at 25 AU resolution. With these ALMA and previous 2008 Submillimeter Array (SMA) data, the proper motion of 8 knots within 250 AU of the central source is found to be 0.068" per year (102 km/s), consistent with previous measurements in the outer jet. At 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 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.