Detection of HF emission from the Orion Bar

Abstract

The clumpy density structure of photon-dominated regions is well established, but the physical properties of the clumps and of the surrounding interclump medium are only approximately known. The aim of this paper is to constrain the physical and chemical conditions in the Orion Bar, a prototypical nearby photon-dominated region. We present observations of the HF J=1-0 line, which appears in emission toward the Orion Bar, and compare the brightness of the line to non-LTE radiative transfer calculations. The large width of the HF line suggests an origin of the emission in the interclump gas, but collisional excitation by H2 in the interclump gas underpredicts the observed line intensity by factors of 3-5. In contrast, an origin of the line in the dense clumps requires a density of ~10^9 cm^-3, 10-100 times higher than previous estimates, which is unlikely. However, electron impact excitation reproduces our observations for T = 100 K and n(e) = 10 cm^-3, as expected for the interclump gas. We conclude that HF emission is a signpost of molecular gas with a high electron density. Similar conditions may apply to active galactic nuclei where HF also appears in emission.

Accepted for publication in A&A; 4 pages, 1 figure

References in corpus (8)

• The distance to the Orion Nebula
• Polycylcic Aromatic Hydrocarbons (PAH's) in dense cloud chemistry
• Observations of Arp 220 using Herschel-SPIRE: An Unprecedented View of the Molecular Gas in an Extreme Star Formation Environment
• Discovery of Interstellar CF+
• Herschel/HIFI Observations of Hydrogen Fluoride Toward Sagittarius B2(M)
• HIFI detection of HF in the carbon star envelope IRC +10216
• Radiative transfer and molecular data for astrochemistry (Review)
• Discovery of Hydrogen Fluoride in the Cloverleaf Quasar at z = 2.56

Cited by in corpus (25)

• Astrochemistry of dust, ice and gas: introduction and overview
• Interstellar Hydrides
• Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220
• The chemistry of ions in the Orion Bar I. - CH+, SH+, and CF+: The effect of high electron density and vibrationally excited H2 in a warm PDR surface
• A Herschel Space Observatory Spectral Line Survey of Local Luminous Infrared Galaxies from 194 to 671 Microns
• A complete model of CH+ rotational excitation including radiative and chemical pumping processes
• Herschel/SPIRE Sub-millimeter Spectra of Local Active Galaxies
• Spectroscopically resolved far-IR observations of the massive star-forming region G5.89--0.39
• Spatially extended OH+ emission from the Orion Bar and Ridge
• The IRAM-30m line survey of the Horsehead PDR: I. CF+ as a tracer of C+ and a measure of the Fluorine abundance
• Electron-impact excitation of diatomic hydride cations I: HeH${}^{+}$, CH${}^{+}$, ArH${}^{+}$
• Herschel / HIFI spectral line survey of the Orion Bar - Temperature and density differentiation near the PDR surface
• Hydrogen Fluoride toward Luminous Nearby Galaxies: NGC 253 and NGC 4945
• The water line emission and ortho-to-para ratio in the Orion Bar photon-dominated region
• Direct estimation of electron density in the Orion Bar PDR from mm-wave carbon recombination lines
• The ramp-up of interstellar medium enrichment at z>4
• Origin of hydrogen fluoride emission in the Orion Bar: An excellent tracer for CO-dark H2 gas clouds
• Collisional quenching of highly rotationally excited HF
• Molecular tracers of radiative feedback in Orion (OMC-1). Widespread CH+ (J=1-0), CO (10-9), HCN (6-5), and HCO+ (6-5) emission
• Dents in the Veil: Protostellar feedback in Orion
• Three-dimensional distribution of hydrogen fluoride gas toward NGC6334 I and I(N)
• Etching glass in the early Universe: Luminous HF and water emission in a QSO-SMG pair at z=4.7
• The first results from the Herschel-HIFI mission
• Using deuterated H3+ and other molecular species to understand the formation of stars and planets
• First detection of CF${}^{+}$ in the Large Magellanic Cloud