Near‐Infrared Spectroscopy of Photodissociation Regions: The Orion Bar and Orion S

Abstract

We have obtained moderate-resolution (R D 3000) spectra of the Orion bar and Orion S regions at J (1.25 km), H (1.64 km), and K (2.2 km). Toward the bar, the observations reveal a large number of emission lines that, when compared to model predictions of Draine & Bertoldi, are indicative of a H 2 high-density photodissociation region (PDR) cm~3, s \ 105, K) rather than of (n H \ 106 T 0 \ 1000 shocked material. Behind the bar and into the molecular cloud, the spectrum again matches well H 2 with that predicted for a dense PDR cm~3) but with a lower temperature K) and (n H \ 106 (T 0 \ 500 UV Ðeld strength (s \ 104). The spectrum and stratiÐcation of near-IR emission lines (O I, H I, H 2 [Fe II], [Fe III], near Orion S imply the presence of a dense PDR with an inclined geometry in this H 2) region cm~3, s \ 105, K). The extinction measurements toward the bar (n H \ 106 T 0 \ 1500 (A K D 2.6) and Orion S emission regions are much larger than expected from either face-on (A K D 2.1) H 2 or edge-on homogeneous PDRs, indicating that clumps may signiÐcantly a †ect the (A K D 0.1) (A K D 1) structure of the PDRs. In addition, we have observed the strongest D30 near-IR He I emission lines, many of which have not been detected previously. There is good agreement between most observed and theoretical He I line ratios, while a few transitions with upper levels of n 3P (particularly 4 3PÈ3 3S 1.2531 km) are enhanced over strengths expected from collisional excitation. This e †ect is possibly due to opacity in the UV series n 3PÈ2 3S. We also detect several near-IR [Fe II] and [Fe III] transitions with line ratios indicative of low densities cm~3), whereas recent observations of optical [Fe II] emission imply the presence (n e D 103È104 of high-density gas cm~3). These results are consistent with a model in which high-density, (n e D 106 partially-ionized gas is the source of the iron transitions observed in the optical, while low-density, fully-ionized material is responsible for the near-IR emission lines. Subject headings : infrared : ISM : lines and bands È ISM : individual (Orion Nebula) È ISM : molecules È ISM : structure