The H 2 structure of OMC – 1 ?

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 (<∼ 15 000K) 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∼1017 cm from the IRc2 area. The projected length of the linear features is a few times 1017 cm. Some of the H2 fingers seem to be associated with fast moving H2O masers. H2 emission from higher levels (>∼ 19 000K) shows a different nebular structure: Two nebulosities, one around BN which is bipolar and the other one extending north–eastwards of IRc2 in the Peak 2 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 Peak 2 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 thewakes of dense “bullets” ploughing through the surroundingmolecular 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 Send offprint requests to: H. Schild ? Based on observations obtained at the European Southern Observatory, La Silla, Chile and the United Kingdom infrared Telescope on Mauna Kea, Hawaii 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–Pérot images of OMC–1 in the light of selected K band transitions of the molecular ion H3. These include 2ν2 2 R(6) at 2.0933μm, 2ν 2 2 Q(4) at 2.1944μm, 2ν2 2 R(8) at 2.1342μm, 2ν 2 2 P(5) at 2.2028μm and 2ν 2 2 P(3) at 2.2039μm. No H3 was detected. We also observed Peak 2 spectroscopically in the L band but again without a detection of H3. These observations set a probable upper limit of the H + 3 concentration of 8.5× 109 cm−2 in OMC–1.