2007A&A...474..941G

Optical and near-infrared spectroscopy of molecular hydrogen in interstellar shocks provide a very powerful probe of the physical conditions that prevail in interstellar shocks. Integral-field spectroscopy of H₂ in the optical wavelength region and complementary long-slit near-infrared spectroscopy towards HH91A are used to characterize the ro-vibrational population distribution among H₂ levels with excitation energies up to 30000cm^–1. The detection of some 200 ro-vibrational lines of molecular hydrogen ranging between 7700Å and 2.3µm is reported. Emission lines which arise from vibrational levels up to v'=8 are detected. The H₂ emission arises from thermally excited gas where the bulk of the material is at a temperature of 2750K and where 1% is at 6000K. The total column density of shocked molecular hydrogen is N(H₂)=10¹⁸cm^–2. Non-thermal excitation scenarios such as UV fluorescence do not contribute to the H₂ excitation observed towards HH91A. The emission of molecular hydrogen towards HH91A is explained in terms of a slow J-shock that propagates into a low-density medium, that has been swept up by previous episodes of outflows that have occurred in the evolved HH90/91 complex.