Far-Infrared detection of neutral atomic oxygen toward the Horsehead Nebula

Abstract

We present the first detection of neutral atomic oxygen (3P_1-3P_2 fine structure line at ~63um) toward the Horsehead photodissociation region (PDR). The cloud has been mapped with the Spitzer Space Telescope at far-IR (FIR) wavelengths using MIPS in the spectral energy distribution (SED) mode. The [OI]63um line peaks at the illuminated edge of the cloud at AV~0.1-0.5 (inward the gas becomes too cold and outward the gas density drops). The luminosity carried by the [OI]63um line represents a significant fraction of the total FIR dust luminosity (I_63/I_FIR~4x10^-3). We analyze the dust continuum emission and the nonlocal OI excitation and radiative transfer in detail. The observations are reproduced with a gas density of n_H~10^4 cm^-3 and gas and dust temperatures of T_k~100 K and T_d~30 K. We conclude that the determination of the OI 3P_J level populations and emergent line intensities at such ``low'' densities is a complex non-LTE problem. FIR radiative pumping, [OI]63um subthermal emission, [OI]145um suprathermal and even maser emission can occur and decrease the resulting [OI]63/145 intensity ratio. The Herschel Space Observatory, observing from ~55 to 672um, will allow us to exploit the diagnostic power of FIR fine structure lines with unprecedented resolution and sensitivity.

Accepted for publication in ApJ Letters (editorial corrections included)

References in corpus (8)

• A PDR-Code Comparison Study
• Low sulfur depletion in the Horsehead PDR
• The penetration of FUV radiation into molecular clouds
• HCO mapping of the Horsehead : Tracing the illuminated dense molecular cloud surfaces
• The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds
• Aromatic emission from the ionised mane of the Horsehead nebula
• Dust processing in photodissociation regions - Mid-IR emission modelling
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Cited by in corpus (16)

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• First detection of interstellar S2H
• H2CO in the Horsehead PDR: Photo-desorption of dust grain ice mantles
• Observing a column-dependent zeta in dense interstellar sources: the case of the Horsehead Nebula
• Complex organic molecules in strongly UV-irradiated gas
• Chemical complexity in the Horsehead photodissociation region
• Abundances of sulphur molecules in the Horsehead nebula. First NS+ detection in a photodissociation region
• Gas phase Elemental abundances in Molecular cloudS (GEMS). IV. Observational results and statistical trends
• [CII] emission from L1630 in the Orion B molecular cloud
• The IRAM-30m line survey of the Horsehead PDR: I. CF+ as a tracer of C+ and a measure of the Fluorine abundance
• Probing the role of polycyclic aromatic hydrocarbons in the photoelectric heating within photodissociation regions
• A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry
• An observational investigation of the identity of B11244 ($l$-C${}_3$H${}^{+}$/C${}_3$H${}^{-}$)
• Dust temperature and time-dependent effects in the chemistry of photodissociation regions
• Spatial distribution of organics in the Horsehead nebula: Signposts of chemistry driven by atomic carbon