2000A&A...356..705R

Using the Short-Wavelength-Spectrometer on the Infrared Space Observatory (ISO), we obtained near- and mid-infrared spectra toward the brightest H₂ emission peak of the Orion OMC-1 outflow. A wealth of emission and absorption features were detected, dominated by 56 H₂ ro-vibrational and pure rotational lines reaching from H₂ 0-0 S(1) to 0-0 S(25). The spectra also show a number of H I recombination lines, atomic and ionic fine structure lines, and molecular lines of CO and H₂O. Between 6 and 12 µm the emission is dominated by PAH features. The extinction toward the molecular and atomic line emitting regions is estimated from relative line intensities, and it is found that the H₂ emission arises from within the OMC-1 cloud at an average K-band extinction of 1.0mag, whereas the atomic hydrogen emission and much of the fine structure emission comes from the foreground H II region and its bounding photodissociation front. The total H₂ luminosity in the ISO-SWS aperture is estimated at (17±5)L_☉, and extrapolated to the entire outflow, (120±60)L_☉. The H₂ level column density distribution shows no signs of fluorescent excitation or a deviation from an ortho-to-para ratio of three. It shows an excitation temperature which increases from about 600K for the lowest rotational and vibrational levels to about 3200K at level energies E(v,J)/k>14000K. No single steady state shock model can reproduce the observed H₂ excitation. The higher energy H₂ levels may be excited either thermally in non-dissociative J-shocks, through non-thermal collisions between fast ions and molecules with H₂ in C-shocks, or they are pumped by newly formed H₂ molecules. The highest rotational levels may be populated by yet another mechanism, such as the gas phase formation of H₂ through H^–.