Sub-arcsecond near-infrared images reveal the density structure of the Orion Bar

Abstract

The Orion Bar is one of the best studied photo-dissociation regions. Both the fact that it is close to us and that it is seen edge-on allow detailed analysis of the interaction of the newly formed stars in the Orion Nebula with the molecular cloud. Previous data sets from this region have provided a great deal of indirect evidence for a structure of low and high density 'clumps' on small scales (~0.1pc). A clumpy structure for the Bar is required in order to explain both the spatial extent of emission from molecular species, for example H2, and observed line ratios such as the CO7-6 / 14-13 ratio. Our observations use the strong density dependence of the ratio of the 1 - 0 S(1) and 2 - 1 S(1) lines of molecular hydrogen to explore the physical structure of the Bar. Images taken with the infrared camera, IRCAM3, on UKIRT show linear details to scales of ~0.5arcsecs extending over tens of arcseconds into the molecular cloud. The ratio of the line strengths from the two images allows the high density 'clumps' or filaments and the lower density 'inter-clump' medium to be identified. Analysis of these data will clarify the significance, or otherwise, of the clumps/filaments in controlling the chemistry and physics of the interstellar medium in Orion.