Near-infrared spectroscopy of photodissociation regions: The Orion bar and Orion S

We have obtained moderate-resolution (R ∼ 3000) spectra of the Orion bar and Orion S regions at J (1.25 μm), H (1.64 μm), and K (2.2 μm). Toward the bar, the observations reveal a large number of H₂ emission lines that, when compared to model predictions of Draine & Bertoldi, are indicative of a high-density photodissociation region (PDR) (n_H = 10⁶ cm^-3, χ = 10⁵, T₀ = 1000 K) rather than of shocked material. Behind the bar and into the molecular cloud, the H₂ spectrum again matches well with that predicted for a dense PDR (n_H = 10⁶ cm^-3) but with a lower temperature (T₀ = 500 K) and UV field strength (χ = 10⁴). The H₂ spectrum and stratification of near-IR emission lines (O I, H I, [Fe II], [Fe III], H₂) near Orion S imply the presence of a dense PDR with an inclined geometry in this region (n_H = 10⁶ cm^-3, χ = 10⁵, T₀ = 1500 K). The extinction measurements toward the bar (A_K ∼ 2.6) and Orion S (A_K ∼ 2.1) H₂ emission regions are much larger than expected from either face-on (A_K ∼ 0.1) or edge-on (A_K ∼ 1) homogeneous PDRs, indicating that clumps may significantly affect the structure of the PDRs. In addition, we have observed the strongest ∼ 30 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 n³P (particularly 4³P-3³S 1.2531 μm) are enhanced over strengths expected from collisional excitation. This effect is possibly due to opacity in the UV series n³P-2³S. We also detect several near-IR [Fe II] and [Fe III] transitions with line ratios indicative of low densities (n_e ∼ 10³-10⁴ cm^-3), whereas recent observations of optical [Fe II] emission imply the presence of high-density gas (n_e ∼ 10⁶ cm^-3). These results are consistent with a model in which high-density, partially-ionized gas is the source of the iron transitions observed in the optical, while low-density, fullyionized material is responsible for the near-IR emission lines.