2003A&A...412..745R

We propose that some low excitation HH objects might correspond to the head of a pulsed jet moving into a dense environment. We develop an analytic model showing that the head of such a jet will move slowly, producing a low excitation bow shock. At the times at which the successive pulses catch up with the jet head, high excitation emission will be produced, lasting for a time of the order of the cooling timescale of the material heated in the pulse/jet head interaction. In this way, the jet head has ``flashes'' of high excitation emission superimposed on a ``quiescent'', low excitation emission.