1997A&A...318..608S

We have obtained narrow-band images of the Orion Molecular Cloud OMC-1 in various infrared transitions of molecular hydrogen. The molecular emission from levels with low excitation energies (≲15000K) is made up of numerous linear, jet-like structures. Their collimation factor is typically about 10 and there is no appreciable outward broadening. Most of these structures are arranged radially and seem to originate from an area close to IRc2. They however only become detectable at a projected distance of ∼10¹⁷cm from the IRc2 area. The projected length of the linear features is a few times 10¹⁷cm. Some of the H₂ fingers seem to be associated with fast moving H₂O masers. H₂ emission from higher levels (>19000K) shows a different nebular structure: Two nebulosities, one around BN which is bipolar and the other one extending north-eastwards of IRc2 in the Peak2 region, are detected. Images in various continuum wavelengths show the same pattern as the H₂ emission from high levels. From these images and in conjunction with long slit K and L band spectra of the Peak2 region we obtain a map of the molecular temperature in OMC-1. The resulting temperature distribution shows little spatial structure on small or intermediate distance scales. It does not show the jet-like structures anymore but displays radial symmetry. The highly structured H₂ emission is mainly due to strong density rather than temperature variations. The observations suggest that the H₂ emission in the linear structures comes from a thin sheet with a thickness of less than one hundredth of the jet width. It is most likely due to molecular material swept up or entrained into the jet from the outside. The visual impression provided by high resolution images is that of very dynamic processes disrupting a molecular cloud. The observed structure has been described by the wakes of dense "bullets" ploughing through the surrounding molecular gas. The smooth and only radially variable temperature distribution can however best be interpreted in terms of instabilities in the interaction zone of a stellar wind which collides with surrounding dense molecular material. A thin layer of shocked material is subject to rapidly growing instabilities which may be responsible for the observed linear structures. We draw attention to the fact that the "thin layer instability" in rapidly cooling radiative shock zones may be more important than Rayleigh-Taylor instabilities. We have also obtained Fabry-Perot images of OMC-1 in the light of selected K band transitions of the molecular ion H₃⁺. These include 2ν₂²R(6) at 2.0933µm, 2ν₂²Q(4) at 2.1944µm, 2ν₂²R(8) at 2.1342µm, 2ν₂²P(5) at 2.2028µm and 2ν₂²P(3) at 2.2039µm. No HSD3p was detected. We also observed Peak2 spectroscopically in the L band but again without a detection of HSD3p. These observations set a probable upper limit of the HSD3p concentration of 8.5x10⁹cm^–2 in OMC-1.