Abstract
Aims. 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. Methods. Integral-field spectroscopy of H 2 in the optical wavelength region and complementary long-slit near-infrared spectroscopy towards HH91A are used to characterize the ro-vibrational population distribution among H 2 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 A and 2.3 μ m is reported. Emission lines which arise from vibrational levels up to $v' = 8$ are detected. The H 2 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 (H 2 ) = 10 18 cm -2 . Non-thermal excitation scenarios such as UV fluorescence do not contribute to the H 2 excitation observed towards HH91A. Conclusions. 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.
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