25 AU Angular Resolution Observations of HH 211 with ALMA : Jet Properties and Shock Structures in SiO, CO, and SO

TL;DR

This study uses high-resolution ALMA observations to analyze the inner jet structure of HH 211, revealing detailed shock features, sub-knots, and jet dynamics, advancing understanding of protostellar jet properties.

Contribution

It provides the first high-resolution imaging of inner jet substructures and shock features in HH 211, and interprets knot variability with a shorter ejection period.

Findings

Proper motion of knots is consistent with previous measurements.

Inner jet contains at least 5 sub-knots with a 4.5-year ejection period.

Resolved shocks and layered chemical structures in the jet.

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(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.