Quantitative optical and near-infrared spectroscopy of H2 towards HH91A

Aims. Optical and near-infrared spectroscopy of molecular hydrogen in interstellar shocks provide a very powerful probe to the physical conditions that prevail in interstellar shocks. Methods. Integral-field spectroscopy of H2 in the optical wavelength region and complementary long-slit near-infrared spectroscopy towards HH91A is used to characterize the ro-vibrational population distribution among H2 levels with excitation energies up to 30 000 cm−1. Results. The detection of some 200 ro-vibrational lines of molecular hydrogen ranging between 7700 Å and 2.3 μm 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 2750 K and where 1% is at 6000 K. The total column density of shocked molecular hydrogen is N(H2) = 1018 cm−2. Non-thermal excitation scenarios such as UV fluorescence do not contribute to the H2 excitation observed towards HH91A. Conclusions. The emission of molecular hydrogen towards HH91A is explained in terms of a slow J-shock which propagates into a low-density medium which has been swept-up by previous episodes of outflows which have occurred in the evolved HH90/91