KINEMATICS AND EVOLUTION OF THE HH-34 COMPLEX

We discuss the kinematics and evolution of the HH 34 complex based on accurate proper motions and radial velocities. We find that the bow shocks on opposite sides of the source recede from it at highly supersonic velocities (approximately 330 km s-1) along a tightly defined axis inclined at an angle of approximately 30-degrees to the plane of the sky. The space velocity we determine for the HH 34 bow shock is substantially greater than its shock velocity, approximately 160 km s-1, derived spectroscopically. There observations can only be reconciled if HH 34 advances into a medium already moving away from the source at a velocity approximately 170 km s-1. We have detected faint emission, at the appropriate radial velocity, from the region just ahead of HH 34. The existence of multiple bow shocks in the approaching lobe of the flow indicates that the driving source has experienced repeated episodes of enhanced mass loss at intervals approximately 400 years. We speculate that each successive outflow imparts an impulse to the ambient medium incrementally accelerating it to a large fraction of the outflow velocity. Consistent with this idea the flux of momentum now carried by the moving medium ahead of HH 34 is found to be comparable to that in the extant outflows. The knots in the HH 34 jet also have substantial space motions (approximately 220 km s-1) and thus cannot be stationary crossing shocks. We find that the relative brightness of these knots changes with time. Comparing this temporal change with the spatial trend in knot brightness suggests that the knots represent a train of similar shock waves, the physical parameters of which evolve as they propagate along the jet. The width of the knots begins to increase, and their electron density to decrease, precisely at the point where they begin to fade. The behavior of the knots is discussed in the context of current models for knot production in astrophysical jets.