Optically thick Herbig-Haro jets in photoionized regions

Over the last few years, a number of Herbig-Haro (HH) jets have been discov ered within the confines of H II regions. These jets apparently are ejected from low mass stars that are similar to the sources of other "normal" HH j ets, but the flows show some special characteristics which are a result of the photoionization due to the ionizing photons emitted by the star that ex cites the H II region. In this paper, we present HH jet models including an external, photoionizing radiation field, and study the effects of increasi ng ionizing photon flxes F-* on the structure of the head of the jet. We fi nd that the morphology, the emission line fluxes and the line ratios change drastically as a function of increasing F-*. These results give general gu idelines for deciding whether an HH jet actually does lie within an H II re gion, based on the morphology and spectroscopic characteristics of the obje ct.