Abstract
Knots in some of the jet-like Herbig-Haro (HH) objects have been successfully modeled as ``internal working surfaces'' resulting from a time-variability of the ejection velocity. The emission from these knots appears to be almost invariably dominated by the emission from the bow shock, and has at most a small contribution from the jet shock (or Mach disk). As has been previously pointed out, this observational result implies that the flow upstream of the working surface is considerably denser than the downstream flow. This density jump could either be generated by the jet flow itself, or it might be the result of a time-dependence in the ejection density. We study a simple, semi-analytic model for the propagation of an internal working surface. With this model, we show that jets ejected with a sinusoidal velocity variability (and with a time-independent density) always develop a strong up- to down-stream density jump across internal working surfaces. From this result, we conclude that the observed density jumps can be straightforwardly explained without recurring to a time-dependent ejection density.
