Near-Infrared H2 and Associated O0 and C+ Emission from Dense Photon-dominated Regions

We have observed near-infrared H2 vibrational line emission from dense (n ≳ 104 cm-3) photon-dominated regions (PDRs) along the Orion A molecular cloud “ridge” and toward the Orion B molecular cloud near NGC 2024. Specifically, we have observed the 1.601 μm v = 6-4 Q(1), 2.121 μm v = 1-0 S(1), and 2.247 μm v = 2-1 S(1) lines of H2. To assess the physical state of the PDRs, we combine our H2 data with observations of the [O I] 63 μm, [O I] 146 μm, and [C II] 158 μm atomic fine-structure lines along the same lines of sight. We use our data to study how the character of the H2 emission depends on the PDR conditions. For the range of densities (104-105 cm-3) and incident far-ultraviolet fluxes (103-105 times the mean interstellar field) in the PDRs, we find that the relative H2 line strengths are characteristic of both low gas density and high gas density UV-excited emission. In Orion, we observe direct evidence of the transition from the (low-density) regime in which the H2 vibrational cascade is dominated by radiative transitions to the (high-density) regime in which collisions influence the vibrational cascade. Our observations show that, for Orion-type abundances and physical conditions, collisions begin to influence the H2 ro-vibrational line ratios near a gas density of 5 × 104 cm-3.

[1]  Near-Infrared Spectra of IC 59/63 and NGC 1535: Comparing Infrared and Ultraviolet Observations of H2 , 1997 .

[2]  H. Kataza,et al.  Fabry-Perot Imaging of H 2 1--0 S(1), 2--1 S(1), and Brackett-Gamma Emission in the Orion Nebula , 1996 .

[3]  A. L. Betz,et al.  Heterodyne Spectroscopy of the 63 μm O I Line in M42 , 1996, astro-ph/9603136.

[4]  Structure of Stationary Photodissociation Fronts , 1996, astro-ph/9603032.

[5]  D. Jaffe,et al.  Ultraviolet-excited H 2 : A Comparison with Large-Scale Tracers of Molecular Clouds , 1996 .

[6]  G. Rieke,et al.  High-Resolution Near-Infrared Spectroscopy of Hubble 12 , 1996 .

[7]  P. Martin,et al.  Analytic Temperature Dependences for a Complete Set of Rate Coefficients for Collisional Excitation and Dissociation of H2 Molecules by H Atoms , 1995 .

[8]  Daniel T. Jaffe,et al.  A new Fabry-Perot spectrometer for observations of diffuse near-infrared line emission , 1995 .

[9]  Daniel T. Jaffe,et al.  H2 emission as a tracer of molecular hydrogen: Large-scale observations of Orion , 1994 .

[10]  A. Poglitsch,et al.  Ultraviolet-illuminated molecular cloud boundaries: Extended (C II) 158 micron emission toward L1630 , 1994 .

[11]  A. Chrysostomou,et al.  Physical conditions in photodissociation regions: M17 northern bar , 1993 .

[12]  M. Mountain,et al.  Pure fluorescent H2 emission from Hubble 12 , 1993 .

[13]  E. Serabyn,et al.  H_2 morphology of young planetary nebulae , 1993 .

[14]  A. Tielens,et al.  Models of clumpy photodissociation regions , 1993 .

[15]  A. Poglitsch,et al.  158 micron forbidden C II mapping of the Orion molecular cloud , 1993 .

[16]  M. Hayashi,et al.  Molecular Cloud Cores in the Orion A Cloud. I. Nobeyama CS (1--0) Survey , 1993 .

[17]  D. Jaffe,et al.  Extended CO(7→6) emission from warm gas in Orion , 1993, astro-ph/9301009.

[18]  T. D. Wilson,et al.  A spatially resolved photodissociation region in the planetary nebula NGC 7027 , 1993 .

[19]  M. Burton,et al.  Fluorescent molecular hydrogen line emission in the far-red , 1992 .

[20]  A. Tielens,et al.  Low-Density Photodissociation Regions , 1991 .

[21]  I. Gatley,et al.  Infrared fluorescence of H2 in NGC 6240 - A starburst origin for the H2 luminosity , 1991 .

[22]  J. Baldwin,et al.  Physical conditions in the Orion Nebula and an assessment of its helium abundance , 1991 .

[23]  J. Lacy,et al.  Detection of low-J pure-rotational emission from H2 in the Orion Bar region - Evidence for small-scale clumpiness , 1991 .

[24]  N. Geis,et al.  The MPE/UCB far-infrared imaging Fabry-Perot interferometer (FIFI) , 1990 .

[25]  A. Tielens,et al.  Line emission from clumpy photodissociation regions , 1990 .

[26]  A. Sternberg,et al.  The infrared response of molecular hydrogen gas to ultraviolet radiation: high-density regions , 1989 .

[27]  I. Gatley,et al.  Infrared spectroscopy of interstellar molecular hydrogen - decomposition of thermal and fluorescent components , 1989 .

[28]  A. Sternberg.  The infrared response of molecular hydrogen gas to ultraviolet radiation: a scaling law , 1988 .

[29]  D. Lester,et al.  Detection of fluorescent molecular hydrogen emission in the planetary nebula Hubble 12 , 1988 .

[30]  L. Mundy,et al.  High-resolution images of the Orion molecular ridge in the CS J = 2-1 transition , 1988 .

[31]  J. Black,et al.  Fluorescent excitation of interstellar H2 , 1987 .

[32]  I. Gatley,et al.  Level population and para/ortho ratio of fluorescent H2 in NGC 2023 , 1987 .

[33]  H. Okuda,et al.  Fluorescent Molecular Hydrogen Emission from the Reflection Nebula NGC 2023 , 1987 .

[34]  R. Wilson,et al.  Filamentary structure in the Orion molecular cloud , 1986 .

[35]  John H. Black,et al.  Comprehensive models of diffuse interstellar clouds : physical conditions and molecular abundances , 1986 .

[36]  K. Sellgren.  Ultraviolet-pumped Infrared Fluorescent Molecular Hydrogen Emission in Reflection Nebulae , 1986 .

[37]  L. Mundy,et al.  Protostellar Condensations within the Orion Ridge , 1986 .

[38]  I. Gatley,et al.  The molecular hydrogen emission associated with the Orion bright bar , 1985 .

[39]  A. Tielens,et al.  Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region , 1985 .

[40]  A. Tielens,et al.  Photodissociation regions. II. A model for the Orion photodissociation region. , 1985 .

[41]  C. Lada,et al.  NGC 2024 : Far-infrared and radio molecular observations. , 1984 .

[42]  R. Few,et al.  Formaldehyde and OH in the Orion molecular clouds , 1983 .

[43]  C. Townes,et al.  Small rotating clouds of stellar mass in Orion molecular cloud 1 , 1983 .

[44]  B. Anthony-Twarog.  The H-beta distance scale for B stars: the Orion association. , 1982 .

[45]  H. Thronson,et al.  The nature of NGC 2024 - Near-infrared spectroscopy of IRS 1 and IRS 2 , 1981 .

[46]  H. Moseley,et al.  Observations of five moderate-luminosity far-infrared sources in Orion and Monoceros. , 1978 .

[47]  B. Draine.  Photoelectric heating of interstellar gas , 1978 .

[48]  J. Launay,et al.  Molecular collision processes. II - Excitation of the fine-structure transition of C/+/ in collisions with H2 , 1977 .

[49]  È. Roueff,et al.  Fine-structure excitation of ground-state C+ ions by hydrogen atoms , 1977 .

[50]  I. Gatley,et al.  One arc-minute resolution maps of the Orion Nebula at 20, 50, and 100 microns , 1976 .

[51]  J. Black,et al.  Interstellar H$sub 2$: The population of excited rotational states and the infrared response to ultraviolet radiation , 1976 .