Near-infrared spectroscopy of photodissociation regions

The Orion bar and Orion S

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

Research output: Contribution to journalArticle

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Abstract

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
Volume499
Issue number2 PART I
DOIs
StatePublished - Jan 1 1998

Fingerprint

infrared spectroscopy
photodissociation
near infrared
low density materials
ionized gases
gas density
clumps
stratification
opacity
molecular clouds
gas
field strength
extinction
iron
geometry
prediction
predictions
excitation

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Luhman, Kevin ; Engelbracht, C. W. ; Luhman, M. L. / Near-infrared spectroscopy of photodissociation regions : The Orion bar and Orion S. In: Astrophysical Journal. 1998 ; Vol. 499, No. 2 PART I. pp. 799-809.
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abstract = "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.",
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Near-infrared spectroscopy of photodissociation regions : The Orion bar and Orion S. / Luhman, Kevin; Engelbracht, C. W.; Luhman, M. L.

In: Astrophysical Journal, Vol. 499, No. 2 PART I, 01.01.1998, p. 799-809.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Near-infrared spectroscopy of photodissociation regions

T2 - The Orion bar and Orion S

AU - Luhman, Kevin

AU - Engelbracht, C. W.

AU - Luhman, M. L.

PY - 1998/1/1

Y1 - 1998/1/1

N2 - 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.

AB - 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.

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