Optical emission from shocks. IV. The Herbig-Haro objects.

Extensive absolute spectrophotometry of HH objects has been obtained with the 3.9 m AAT and the 1.9 m Mount Stromlo telescopes. Systematic trends in their spectral characteristics have been found. All the observations can be explained in a quantitative manner using a theory of radiating shock waves traveling into a partially preionized medium. There are probably two classes of stellar objects driving the shocks. These are either B-type protostars in the process of breaking up optically thick dust cocoons, or FU Orionis-type stars losing mass. In either case, high luminosities and/or high mass-loss rates are required. If the stellar object is an FU Orionis object, these must be maintained over time scales of order 100 years.