Letter to the Editor

Interstellar OH⁺, H₂O⁺ and H₃O⁺ along the sight-line to G10.6–0.4 ^*,**

¹ LERMA, CNRS, Observatoire de Paris and ENS, France e-mail: maryvonne.gerin@ens.fr
² Chalmers University of Technology, Göteborg, Sweden
³ Centro de Astrobiología, CSIC-INTA, Madrid, Spain
⁴ Depts. of Physics, Astronomy & Chemistry, Ohio State Univ., USA
⁵ The Johns Hopkins University, Baltimore, MD 21218, USA
⁶ Institut d'Astrophysique Spatiale (IAS), Orsay, France
⁷ JPL, California Institute of Technology, Pasadena, USA
⁸ California Institute of Technology, Pasadena, CA 91125, USA
⁹ Université de Toulouse; UPS; CESR; 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France
¹⁰ CNRS; UMR5187; 31028 Toulouse, France
¹¹ I. Physikalisches Institut, University of Cologne, Germany
¹² Laboratoire d'Astrophysique de Marseille (LAM), France
¹³ Laboratoire d'Astrophysique de Grenoble, France
¹⁴ Nicolaus Copernicus University, Torún, Poland
¹⁵ Inst. of Physical Chemistry, Polish Academy of Sciences, Poland
¹⁶ Tata Institute of Fundamental Research, Mumbai, India
¹⁷ Dept. of Physics & Astronomy, University of Calgary, Canada
¹⁸ European Space Astronomy Centre, ESA, Madrid, Spain
¹⁹ MPI für Extraterrestrische Physik, Garching, Germany
²⁰ MPI für Radioastronomie, Bonn, Germany
²¹ Gemini telescope, Hilo, Hawaii, USA
²² Nicolaus Copernicus Astronomical Center, Poland
²³ SRON Netherlands Institute for Space Research, The Netherlands
²⁴ Sterrewacht Leiden, The Netherlands
²⁵ Deutsches Zentrum für Luft- und Raumfahrt e. V., Raumfahrt-Agentur, Bonn, Germany
²⁶ Osservatorio Astrofisico di Arcetri-INAF- Florence, Italy

Received: 30 March 2010
Accepted: 30 April 2010

Abstract

We report the detection of absorption lines by the reactive ions OH⁺, H₂O⁺and H₃O⁺ along the line of sight to the submillimeter continuum source G10.6–0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH⁺ at 971 GHz, H₂O⁺ at 1115 and 607 GHz, and H₃O⁺ at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6–0.4 as well as in the molecular gas directly associated with that source. The OH⁺ spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H₂O⁺ lines remains lower than 1 in the same velocity range, and the H₃O⁺line shows only weak absorption. For LSR velocities between 7 and 50 km s^-1 we estimate total column densities of N(OH⁺) ≥ 2.5 × 10¹⁴ cm^-2, N(H₂O⁺) ~6 × 10¹³ cm^-2 and N(H₃O⁺) ~4.0 × 10¹³ cm^-2. These detections confirm the role of O⁺ and OH⁺ in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH⁺ by the H₂O⁺  column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.