Letter to the Editor

Herschel/HIFI observations of interstellar OH⁺ and H₂O⁺ towards W49N: a probe of diffuse clouds with a small molecular fraction^*

¹ The Johns Hopkins University, Baltimore, MD 21218, USA e-mail: neufeld@pha.jhu.edu
² Centro de Astrobiología, CSIC-INTA, 28850, Madrid, Spain
³ Chalmers University of Technology, Göteborg, Sweden
⁴ JPL, California Institute of Technology, Pasadena, USA
⁵ California Institute of Technology, Pasadena, CA 91125, USA
⁶ LERMA, CNRS, Observatoire de Paris and ENS, France
⁷ Depts. of Physics, Astronomy & Chemistry, Ohio State Univ., USA
⁸ Institut d'Astrophysique Spatiale (IAS), Orsay, France
⁹ Université Toulouse; UPS; CESR; and CNRS; UMR5187, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
¹⁰ Nicolaus Copernicus University, Torun, Poland
¹¹ Gemini telescope, Hilo, Hawaii, USA
¹² I. Physikalisches Institut, University of Cologne, Germany
¹³ LAM, OAMP, Université Aix-Marseille & CNRS, Marseille, France
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 Department of Physics and Astronomy, University of Waterloo, Canada
²² Osservatorio Astrofisico di Arcetri – INAF, Florence, Italy
²³ Deutsches Zentrum für Luft – und Raumfahrt e. V., Raumfahrt-Agentur, Bonn, Germany
²⁴ SRON Netherlands Institute for Space Research, The Netherlands
²⁵ Sterrewacht Leiden, The Netherlands

Received: 28 May 2010
Accepted: 28 June 2010

Abstract

We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1–0 transition of OH⁺ and the 1115 GHz 1₁₁–0₀₀ transition of ortho-H₂O⁺. The resultant spectra show absorption by ortho-H₂O⁺, and strong absorption by OH⁺, in foreground material at velocities in the range 0 to 70 km s^-1 with respect to the local standard of rest. The inferred OH⁺/H₂O⁺ abundance ratio ranges from ~3 to ~15, implying that the observed OH⁺ arises in clouds of small molecular fraction, in the 2–8% range. This conclusion is confirmed by the distribution of OH⁺ and H₂O⁺ in Doppler velocity space, which is similar to that of atomic hydrogen, as observed by means of 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH⁺/H abundance ratio of a few ×10^-8 suggests a cosmic ray ionization rate for atomic hydrogen of 0.6–2.4×10^-16 s^-1, in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H₃⁺ and other species.