STRUCTURE AND CHEMISTRY OF THE ORION BAR PHOTON-DOMINATED REGION

We present near-infrared spectral line imaging of the photon-dominated region in the Orion bar in the H-2 v = 1-->0 S(1) and v = 2-->1 S(1) vibrational emission lines at an angular resolution of 1.5 '' (0.0033 pc). We also present long slit spectroscopy of these lines at selected positions in the Orion bar, and maps of the (CO)-C-13 J = 3-->2, (CO) -O-18 J = 2-->1, CS J = 5-->4 and (CS)-S-34 J = 3-->2 rotational lines , at angular resolutions of 8 to 17 ''. Combined with existing data, o ur images provide a detailed view of the stratification of abundances and excitation conditions in the edge-on photon-dominated region in th e Orion bar. A thin (approximate to 0.022 pc) layer of atomic hydrogen is located adjacent to the ionization front. The transition to H-2 oc curs 15 '' from the ionization front and is marked by a maximum in the H-2 v = 1-->0 S(1) emission. CO is formed 20 '' from the ionization f ront, and a peak in CS emission occurs slightly deeper inside the clou d. HCO+ J = 1-->0 emission is brightest close to the H-2 v = 1-->0 S(1 ) peak near the illuminated surface. We argue that the HCO+ is produce d by photo-driven reactions in hot gas. Our images show that the neutr al clouds near the bar are very clumpy, containing structure down to t he resolution limit of the observations. The presence of an interclump medium is indicated by the detection of a pervasive purely fluorescen t component to the H-2 emission. The density of hydrogen nuclei in the interclump medium is approximately 10(4) cm(-3), while the CO and CS data indicate densities that are at least a factor of 20 higher. The i mplications of our results for physical models of the structure and ge ometry of the Orion bar are discussed.