Letter to the Editor

Detection of OH⁺ and H₂O⁺ towards Orion KL

¹ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA e-mail: hgupta@jpl.nasa.gov
² Departments of Physics, Astronomy, and Chemistry, Ohio State University, Columbus, OH 43210, USA
³ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁴ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
⁶ Observatorio Astronómico Nacional (IGN), Centro Astronómico de Yebes, Apartado 148. 19080 Guadalajara, Spain
⁷ Microwave Laboratory, ETH Zurich, 8092 Zurich, Switzerland
⁸ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
⁹ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
¹⁰ CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
¹¹ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble, Cedex 9, France
¹² Centro de Astrobiología (CSIC/INTA), Laboratiorio de Astrofísica Molecular, Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁴ LERMA & UMR8112 du CNRS, Observatoire de Paris, 61, Av. de l'Observatoire, 75014 Paris, France

¹⁵ LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
¹⁶ LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France
¹⁷ LUTH, UMR8102, Observatoire de Paris, Meudon, France
¹⁸ University of Massachusetts, Astronomy Dept., 710 N. Pleasant St., LGRT-619E, Amherst, MA 01003-9305 USA
¹⁹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
²⁰ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
²¹ Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
²² Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
²³ Chalmers University of Technology, 412 96 Göteborg, Sweden, Sweden; Department of Astronomy, Stockholm University, 106 91 Stockholm, Sweden
²⁴ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George St., Toronto, ON M5S 3H8, Canada
²⁵ National University of Ireland. Maynooth, Ireland
²⁶ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
²⁷ Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
²⁸ N. Copernicus Astronomical Center, Rabianska 8, 87-100, Torun, Poland
²⁹ MPI für Sonnensystemforschung, D 37191 Katlenburg-Lindau, Germany

Received: 31 May 2010
Accepted: 26 July 2010

Abstract

We report observations of the reactive molecular ions OH⁺, H₂O⁺, and H₃O⁺ towards Orion KL with Herschel/HIFI. All three N = 1-0 fine-structure transitions of OH⁺ at 909, 971, and 1033 GHz and both fine-structure components of the doublet ortho-H₂O⁺ 1₁₁–0₀₀ transition at 1115 and 1139 GHz were detected; an upper limit was obtained for H₃O⁺. OH⁺ and H₂O⁺ are observed purely in absorption, showing a narrow component at the source velocity of 9 km s^-1, and a broad blueshifted absorption similar to that reported recently for HF and para-H₂¹⁸O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH⁺ and H₂O⁺ for the 9 km s^-1 component of 9 ± 3 × 10¹² cm^-2 and 7 ± 2 × 10¹² cm^-2, and those in the outflow of 1.9 ± 0.7 × 10¹³ cm^-2 and 1.0 ± 0.3 × 10¹³ cm^-2. Upper limits of 2.4 × 10¹² cm^-2 and 8.7 × 10¹² cm^-2 were derived for the column densities of ortho and para-H₃O⁺ from transitions near 985 and 1657 GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. The comparatively low column densities may be explained by a higher gas density despite the assumption of a very high ionization rate.