Using the Short-Wavelength-Spectrometer on the Infrared Satellite 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 60 H2 ro-vibrational and pure rotational lines reaching from H2 0–0 S(1) to 0–0 S(25).

The total H2 luminosity in the ISO-SWS aperture is (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 600 K for the lowest rotational and vibrational levels to about 3200 K at level energies E(v, J)/k > 14 000 K.

Introduction

The Orion molecular cloud, OMC-1, located behind the Orion M42 Nebula at a distance of ∼450 pc (Genzel & Stutzki 1989), is the best-studied massive star forming region. This cloud embeds a spectacular outflow arising from some embedded young stellar object, which can possibly be identified as the radio source “I” 0.49 arcsec south of the infrared source IRc2-A (Menten & Reid 1995; Dougados et al. 1993). The outflow shocks the surrounding molecular gas, thereby giving rise to the strongest H2 infrared line emission appearing in the sky. Peak 1 (Beckwith et al. 1978) is the brighter of the two H2 emission lobes of the outflow. Although the outflow has been studied extensively for nearly two decades, the nature of the emission mechanism remains unclear.