Herbig-Haro Jets from Time-Dependent Sources

A considerable amount of effort has been made towards obtaining a theoretical understanding of the collimated, optically detected outflows (Herbig-Haro objects) ejected by young stars. The most clear results have been obtained for the case of the “Herbig-Haro jets”, a loosely defined category which groups the Herbig-Haro (HH) objects with jet-like structures of aligned knots. In particular, it has recently been shown that at least some of the characteristics of the HH jets can be straightforwardly explained in terms of models of jets from variable sources. This paper presents a review of the properties of models of jet flows from sources with a variability in the ejection velocity, in the ejection direction, and with a general velocity+direction variability. Also, a comparison between the observational characteristics of HH jets and the predictions from variable source jet models is carried out.