The HH flow associated with young stellar object HL Tau has an exciting history and even until now the origin of some its features is not clear yet. Flow starts as the relatively faint and narrow jet from HL Tau but at distance of about 20" from the star it abruptly changes direction by about 14 deg and becomes much brighter and appears complex knotty morphology. Knotty structures of Herbig-Haro jets are common phenomena and knowing the origin of such structures is essential for understanding the processes of jet formation. Basically, there are two theoretical approaches: different types of instabilities in stationary flow, and episodic ejection of the matter.
    The first scanning Fabry-Perot interferometry of HL Tau jet revealed the complex kinematical structure (Movsessian et al., 2007) in the jet. In the flow detected compact knots with high (-150 km/s) and bow shape structures in front of those, with low radial velocities (-50 km/s). To study the origin of those structures it is important to have not only radial velocities but tangential velocities also, by the use of proper motion. We use two epoch (2001, 2007) scanning Fabry-Perot observations obtained with SCORPIO focal reducer at SAO RAS 6-m telescope to measure proper motions of the structures with various radial velocities. The most interesting result is that the structural components with low and high radial velocity both have very similar values of tangential velocity nearly 160 km/s.
    This result witnesses to the formation of high and low velocity structures in the HL Tau jet formed by episodic ejection of the matter. In this case we observe emission of high velocity clump of gas and bow shape supersonic wave in front of it. Increase of relative surface brightness of bow-shocks could be the result of abrupt change of the physical conditions of the ambient medium as well as the interaction of a highly collimated flow and the side wind from XZ Tau.
Published: Movsessian T.A., Magakyan T.Yu., Moiseev A.V.,
2012, A&A, 541, A16; arXiv:1203.4074v2 [astro-ph.SR]