Hiding the high excitation in the head of a fast Herbig-Haro jet

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.