High Spectral Resolution Observations of the Molecular Hydrogen Emission in the Orion Molecular Cloud

Observations of line profiles of the vibrationally excited H2 gas in the Orion molecular cloud are presented. The ν = 1 → 0 S(1), ν = 1 → 0 S(0) and ν = 2 → 1 S(1) lines, emitted at a wavelength near 2 μm, have been observed with a spectral resolution of 20 km s-1. The region has been mapped extensively in the ν = 1 → 0 S(1) line with a spatial resolution of 10" and 5", and the line has been monitored at a few positions over a period of 15 months. The profiles of the ν = 1 → 0 S(0) and ν = 2 → 1 S(1) lines have been compared at two positions. The mapping measurements show variations from the periphery where the profiles are narrow, symmetric and have a peak velocity equal to that of the molecular cloud, to the center of the region of emission where the profiles are wide, asymmetric and blueshifted. The line profiles do not appear to vary with time or with the energy of the upper level of the transition. The results are interpreted by a model of a radially expanding flow of gas. From a study of H2 excitation behind a shock front and in a magnetic precursor ahead of a shock wave, it is concluded that the high velocity emission comes from the high velocity gas in the flow, and the low velocity emission comes from gas in the molecular cloud surrounding the flow, with density inhomogeneities contributing to the detailed profiles. Finally the H2 emission is compared to the CO and H2O maser emission and lo the infrared continuum sources. An instrument which incorporates a piezoelectrically scanned Fabry-Pérot interferometer, an InSb detector cooled to ~ 55 K, a rotary chopper, and an offset guider has been built to make these observations. Its operation is described in an appendix.