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A Multi-Epoch SMA Study of the HH 211 Protostellar Jet: Jet Motion and Knot Formation

HH 211 is a highly collimated jet with a chain of well-defined knots, powered by a nearby young Class 0 protostar. We have used 4 epochs (2004, 2008, 2010, and 2013) of Submillimeter Array (SMA) archive data to study the properties of the HH 211 jet in SiO (J=8-7). The jet shows similar reflection-symmetric wiggle structures in all epochs. The wiggle structures can all be fitted by an orbiting jet source model that includes a position shift due to proper motion of the jet, indicating that the wiggle propagates along the jet axis. Thus, this suggests the wiggle is indeed due to an orbital motion of the jet source. Proper motions of the knots are measured by using the peak positions of the knots in four epochs, and they are roughly the same and independent of the distance from the central source. The mean proper motion of the knots is $\sim$ 0.087 arcsec per year, resulting in a transverse velocity of $\sim$ 114 km s$^{-1}$, about 30\% lower than that measured before. Knots BK2 and BK3 have a well-defined linear velocity structure, with the fast jet material upstream to the slow jet material. The gradient of the velocity structure decreases from knot BK2 to BK3. In addition, for each knot, the gradient decreases with time, as the knot propagates away from the central source. These results are both expected if the two knots trace internal shocks produced by a small periodical variation in ejection velocity of the jet.