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

K. L. Luhman, C. W. Engelbracht, M. L. Luhman

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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) (nH = 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 (nH = 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 (nH = 106 cm-3, χ = 105, T0 = 1500 K). The extinction measurements toward the bar (AK ∼ 2.6) and Orion S (AK ∼ 2.1) H2 emission regions are much larger than expected from either face-on (AK ∼ 0.1) or edge-on (AK ∼ 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 (ne ∼ 103-104 cm-3), whereas recent observations of optical [Fe II] emission imply the presence of high-density gas (ne ∼ 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.

Original languageEnglish (US)
Pages (from-to)799-809
Number of pages11
JournalAstrophysical Journal
Issue number2 PART I
StatePublished - 1998

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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