Letter to the Editor

Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: an ubiquitous tracer of molecular gas^*

¹ The Johns Hopkins University, Baltimore, MD 21218, USA e-mail: sonnentr@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, Torún, 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, Torún, Poland
²⁰ European Space Astronomy Centre, ESA, Madrid, Spain
²¹ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA

Received: 29 May 2010
Accepted: 13 July 2010

Abstract

We discuss the detection of absorption by interstellar hydrogen fluoride (HF) along the sight line to the submillimeter continuum sources W49N and W51. We have used Herschel's HIFI instrument in dual beam switch mode to observe the 1232.4762 GHz J = 1–0 HF transition in the upper sideband of the band 5a receiver. We detected foreground absorption by HF toward both sources over a wide range of velocities. Optically thin absorption components were detected on both sight lines, allowing us to measure – as opposed to obtain a lower limit on – the column density of HF for the first time. As in previous observations of HF toward the source G10.6–0.4, the derived HF column density is typically comparable to that of water vapor, even though the elemental abundance of oxygen is greater than that of fluorine by four orders of magnitude. We used the rather uncertain N(CH)–N(H₂) relationship derived previously toward diffuse molecular clouds to infer the molecular hydrogen column density in the clouds exhibiting HF absorption. Within the uncertainties, we find that the abundance of HF with respect to H₂ is consistent with the theoretical prediction that HF is the main reservoir of gas-phase fluorine for these clouds. Thus, hydrogen fluoride has the potential to become an excellent tracer of molecular hydrogen, and provides a sensitive probe of clouds of small H₂ column density. Indeed, the observations of hydrogen fluoride reported here reveal the presence of a low column density diffuse molecular cloud along the W51 sight line, at an LSR velocity of ~24 km s^-1, that had not been identified in molecular absorption line studies prior to the launch of Herschel.