Theoretical interpretation of the apparent deceleration in the HH 34 superjet

The HH 34 superjet shows a steep velocity decrease (from similar to 500 to similar to 100 km s(-1)) over a distance of similar to 2 pc on either side of the central source. We explore whether or not this behaviour could be interpreted as evidence for a slow "turning on" of the ejection velocity of the jet, and find that this is indeed possible, but only for an ejection velocity that has had a dramatic growth over the last similar to 10(4) yr, and is just about to stabilize within the next 4000 yr. We argue that such a time-variability is somewhat unlikely. We then explore a second scenario, in which the slowing down of the HH 34 superjet is modeled as the result of the interaction of a fragmented jet with the surrounding environment. We find that for parameters appropriate for I-II-I 34, this model does appear to reproduce the observed slowing down of the superjet in a natural way. We therefore conclude that the kinematical properties of the HH 34 superjet are most likely to be the result of environmental drag on the propagation of individual jet knots, resulting from the fragmentation of a time-variable, precessing jet.