1994A&A...289..256S

The nature of shocks in star-forming clouds can be deduced from the infrared spectra of molecular hydrogen emission lines. The first clear evidence for a jump (J)-shock within dense molecular gas is found on analysing the published results for the bipolar outflow Herbig-Haro (HH) 90/91 (Gredel et al. 1992). The rotational and vibrational lines of H₂ correspond to J-shocks with standard quiescent-cloud chemistry and any speed in the range 12-21km/s. No alternative shock physics can be applied with satisfaction. The relatively high level of ionisation required to produce a J-shock (as opposed to a C-shock) is consistent with the dense gas being located near the edge of the cloud, thus being directly exposed to the UV of a nearby OB association. Limits to the near infrared [FeII] emission lines are used as a further test. It is shown how to employ these line strengths to distinguish between shock types in HH objects.