ISO-SWS observations of OMC-1: H_2 and fine structure lines

Abstract

Using the Short-Wavelength-Spectrometer on the Infrared Space Observatory (ISO), we obtained near- and mid-infrared spectra toward the brightest H_2 emission peak of the Orion OMC-1 outflow. A wealth of emission and absorption features were detected, dominated by 56 H_2 ro-vibrational and pure rotational lines reaching from H_2 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 H_2O. 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 H_2 emission arises from within the OMC-1 cloud at an average K-band extinction of 1.0 mag, whereas the atomic hydrogen emission and much of the fine structure emission comes from the foreground H ii region and its bounding photodissociation front. H_2 1-0 S(1) emission of the OMC-1 outflow as seen with the NICMOS camera aboard the HST (Schultz et al. 1999). Overlaid are the various apertures of our ISO-SWS observations, which were centered on alpha_2000 . = 5^h 35^m 13.^s 67, delta_2000 . = -5 degr 22 arcmin 8. arcsec 5, with an aperture of 14 arcsec x 20 arcsec for lambda < 12 μm, 14 arcsec x 27 arcsec at 12 to 27.5μm, 20 arcsec x 27 arcsec at 27.5 to 29μm, and 20 arcsec x 33 arcsec at 29 to 45.2μm. The total H_2 luminosity in the ISO-SWS aperture is estimated at (17 +/- 5) L_sun , and extrapolated to the entire outflow, (120 +/- 60) L_sun . The H_2 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 600 K for the lowest rotational and vibrational levels to about 3200 K at level energies E(v,J)/k > 14 000 K. No single steady state shock model can reproduce the observed H_2 excitation. The higher energy H_2 levels may be excited either thermally in non-dissociative J-shocks, through non-thermal collisions between fast ions and molecules with H_2 in C-shocks, or they are pumped by newly formed H_2 molecules. The highest rotational levels may be populated by yet another mechanism, such as the gas phase formation of H_2 through H^-. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.