The H2 velocity structure of inner knots in HH 212: asymmetries and rotation

摘要

High-resolution R~50 000 long-slit spectroscopy of the inner knots of thehighly symmetrical protostellar outflow HH 212 was obtained in the 1-0 S(1)line of H2 at 2.12 micron with a spatial resolution of ~0.45 arcsec. At theresulting velocity resolution of ~6 km s-1, multiple slit oriented observationsof the northern first knot NK1 clearly show double-peaked line profilesconsistent with either a radiative bow shock or dual (forward and reverse)shocks. In contrast, the velocity distribution of the southern first knot SK1remains single-peaked, suggesting a significantly lower jet velocity andpossibly a different density variation in the jet pulses in the southern flowcompared to the northern flow. Comparison with a semi-empirical analyticalmodel of bow shock emission allows us to constrain parameters such as the bowinclination to the line of sight, the bow shock and jet velocities for eachflow. Although a few features are not reproduced by this model, it confirms thepresence of several dynamical and kinematical asymmetries between oppositesides of the HH 212 bipolar jet. The position-velocity diagrams of both knotsexhibit complex dynamics that are broadly consistent with emission from a bowshock and/or jet shock, which does not exclude jet rotation, although a clearsignature of jet rotation in HH 212 is missing. Alternative interpretations ofthe variation of radial velocity across these knots, such as a variation in thejet orientation, as well as for the velocity asymmetries between the flows, arealso considered. The presence of a correlation between flow velocity andcollimation in each flow is suggested.