Kinematics of Herbig-Haro objects in the protostellar outflow L1551 as mapped by Fabry-Perot spectroscopy

We present new velocity-resolved Fabry-Perot images in CS nl lambda 6731 an d H alpha of the blueshifted portion of the protostellar outflow L1551. The se new data isolate the line emission from the reflected continuum and make it possible to visualize the kinematics of the shock waves in the outflow clearly for the first time on large scales with subarcsecond spatial resolu tion. Velocity images of the L1551 jet confirm that a fainter, slower jet l ies a few arcseconds below the main jet. Emission from the main jet decreas es sharply in radial velocity and the emission-line width increases suddenl y as the jet encounters bright knot 3, in agreement with bow shock models. This knot must move into previously ejected material to account for the obs erved radial velocities, supporting the idea that shocks in Herbig-Haro (HH ) flows form as the result of variable velocity ejections from the embedded protostar. However, a velocity gradient observed along the entire edge of the bow shock is spatially larger than expected if the bow shock alone were responsible for all the Line emission. Deviations from the simple model ar e most easily explained if a Mach disk alters the emission and kinematics w ithin the bow shock region, though a precursor to the bow shock is an alter nate possibility. The spatial distribution of radial velocities and emissio n-line widths across HH 29 implies that this object is a slower portion of the outflow currently being overtaken by faster material. New proper-motion images of HH 29 independently confirm this result. The large-scale velocit y structure of the L1551 outflow is complex but can generally be understood if faster material drives shock waves into slower material around the edge of a cavity. A striking circular feature whose center lies near the inters ection of the axis of the bright jet and the cavity resembles similar struc tures in the Orion Nebula and could define a hole through which a fast jet has penetrated. The velocity structure along a string of HH objects to the southwest of HH 29 is consistent with the recent result of Devine et al. th at L1551 NE drives this portion of the outflow. Linear features that cross the L1551 flow may be associated with separate, unrelated jets from the HH 30 region or elsewhere within L1551.