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 H₂ υ = 1→0 S(1) and υ = 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 ¹³CO J = 3→2, C¹⁸O J = 2→1, CS J = 5→4 and C³⁴S J = 3→2 rotational lines, at angular resolutions of 8 to 17″. Combined with existing data, our images provide a detailed view of the stratification of abundances and excitation conditions in the edge-on photon-dominated region in the Orion bar. A thin (≈ 0.022 pc) layer of atomic hydrogen is located adjacent to the ionization front. The transition to H₂ occurs 15″ from the ionization front and is marked by a maximum in the H₂ υ = 1→0 S(1) emission. CO is formed 20″ from the ionization front, and a peak in CS emission occurs slightly deeper inside the cloud. HCO⁺ J = 1→0 emission is brightest close to the H₂ υ = 1→0 S(1) peak near the illuminated surface. We argue that the HCO⁺ is produced by photo-driven reactions in hot gas. Our images show that the neutral clouds near the bar are very clumpy, containing structure down to the resolution limit of the observations. The presence of an interclump medium is indicated by the detection of a pervasive purely fluorescent component to the H₂ emission. The density of hydrogen nuclei in the interclump medium is approximately 10⁴ cm^-3, while the CO and CS data indicate densities that are at least a factor of 20 higher. The implications of our results for physical models of the structure and geometry of the Orion bar are discussed.