0 v 1 2 5 A ug 1 99 8 A 3 - mode , variable velocity jet model for HH 34

Variable ejection velocity jet models can qualitatively explain the appearance of successive working surfaces in Herbig-Haro (HH) jets. This paper presents an attempt to explore which features of the HH 34 jet can indeed be reproduced by such a model. From previously published data on this object, we find evidence for the existence of a 3-mode ejection velocity variability, and then explore the implications of such a variability. From simple, analytic considerations it is possible to show that the longer period modes produce a modulation on the shorter period modes, resulting in the formation of " trains " of multiple knots. The knots observed close to the source of HH 34 could correspond to such a structure. Finally, a numerical simulation with the ejection velocity variability deduced from the HH 34 data is computed. This numerical simulation shows a quite remarkable resemblance with the observed properties of the HH 34 jet.