Letter to the Editor

Strong absorption by interstellar hydrogen fluoride: Herschel/HIFI observations of the sight-line to G10.6–0.4 (W31C)^*

¹ The Johns Hopkins University, Baltimore, MD 21218, USA e-mail: neufeld@pha.jhu.edu
² California Institute of Technology, Pasadena, CA 91125, USA
³ LERMA, CNRS, Observatoire de Paris and ENS, France
⁴ Centro de Astrobiologìa, CSIC-INTA, 28850, Madrid, Spain
⁵ Chalmers University of Technology, Göteborg, Sweden
⁶ Institut d'Astrophysique Spatiale (IAS), Orsay, France
⁷ Université Toulouse; UPS; CESR; and CNRS; UMR5187, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
⁸ Nicolaus Copernicus University, Torun, Poland
⁹ Gemini telescope, Hilo, Hawaii, USA
¹⁰ I. Physikalisches Institut, University of Cologne, Germany
¹¹ JPL, California Institute of Technology, Pasadena, USA
¹² LAM, OAMP, Université Aix-Marseille & CNRS, Marseille, France
¹³ Depts. of Physics, Astronomy & Chemistry, Ohio State Univ., USA
¹⁴ Laboratoire d'Astrophysique de Grenoble, France
¹⁵ Institute of Physical Chemistry, PAS, Warsaw, Poland
¹⁶ MPI für Radioastronomie, Bonn, Germany
¹⁷ Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
¹⁸ Dept. of Physics & Astronomy, University of Calgary, Canada
¹⁹ Nicolaus Copernicus Astronomical Center, Poland
²⁰ European Space Astronomy Centre, ESA, Madrid, Spain
²¹ Institute Université de Bordeaux & CNRS, Bordeaux, France
²² Observatorio Astron. Nacional (IGN) and ALMA, Santiago, Chile

Received: 27 March 2010
Accepted: 12 April 2010

Abstract

We report the detection of strong absorption by interstellar hydrogen fluoride along the sight-line to the submillimeter continuum source G10.6–0.4 (W31C). We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 1232.4763 GHz J = 1–0 HF transition in the upper sideband of the Band 5a receiver. The resultant spectrum shows weak HF emission from G10.6–0.4 at LSR velocities in the range –10 to –3 km s^-1, accompanied by strong absorption by foreground material at LSR velocities in the range 15 to 50 km s^-1. The spectrum is similar to that of the 1113.3430 GHz 1₁₁–0₀₀ transition of para-water, although at some frequencies the HF (hydrogen fluoride) optical depth clearly exceeds that of para-H₂O. The optically-thick HF absorption that we have observed places a conservative lower limit of 1.6×10¹⁴ cm^-2 on the HF column density along the sight-line to G10.6–0.4. Our lower limit on the HF abundance, 6×10^-9 relative to hydrogen nuclei, implies that hydrogen fluoride accounts for between ~30% and 100% of the fluorine nuclei in the gas phase along this sight-line. This observation corroborates theoretical predictions that – because the unique thermochemistry of fluorine permits the exothermic reaction of F atoms with molecular hydrogen – HF will be the dominant reservoir of interstellar fluorine under a wide range of conditions.