Detection of HF emission from the Orion Bar

F. F. S. van der Tak, V. Ossenkopf, Z. Nagy, A. Faure, M. Röllig, E. A. Bergin

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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. Aims. The aim of this paper is to constrain the physical and chemical conditions in the Orion Bar, a prototypical nearby photondominated region. Methods. 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. Results. 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 ∼109cm−3, 10–100 times higher than previous estimates, which is unlikely. However, electron impact excitation reproduces our observations for T=100K and ne=10 cm−3, as expected for the interclump gas. Conclusions. 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.
Original languageEnglish
Article numberL10
Number of pages4
JournalAstronomy & astrophysics
Volume537
DOIs
Publication statusPublished - Jan-2012

Keywords

  • astrochemistry
  • ISM: molecules
  • EXTRAORDINARY SOURCES HEXOS
  • HYDROGEN-FLUORIDE
  • ROTATIONAL-EXCITATION
  • HERSCHEL/HIFI OBSERVATIONS
  • SUBMILLIMETER OBSERVATIONS
  • PHOTODISSOCIATION REGIONS
  • CHEMICAL STRATIFICATION
  • LINE OBSERVATIONS
  • MOLECULAR CLOUDS
  • STAR-FORMATION

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