"Herbig-Haro Jets: The Ejection and the Outside World"

In modeling the hypersonic jets ejected by young stars, one can play with two basic ideas: 1. the observed emission is due to the self-interaction occurring within an outflow ejected in a time-dependent way, 2. the observed structures result from the interaction between the jet and the surrounding environment. Even though adventurous 1.5 decades ago, the first of these possibilities has now become the favored scenario for modeling HH jets. In particular, new observations and modeling of the two well studied objects HH 34 and HH 111 appear to leave little doubt that this scenario is indeed correct at least for some HH jets. The interaction of jets with the "outside world" (once the favored scenario for explaining the observed HH emission, but currently not in the peak of fashion) is an interesting problem, and is clearly occurring in some HH jets. In particular, there are jets that slam into dense obstacles, jets that are swept sideways by a moving environment, and jets that are photo-ionized by external ionizing photon sources. Different examples of these kinds of interactions will be discussed