Letter to the Editor

Herschel/HIFI discovery of interstellar chloronium (H₂Cl⁺) ^*,**

¹ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA e-mail: dcl@caltech.edu
² Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA
³ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁶ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁷ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
⁸ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, Floirac, France
⁹ INAF - Istituto di Fisica dello Spazio Interplanetario, Roma, Italy
¹⁰ Institute of Astronomy, ETH-Zurich, Zurich, Switzerland
¹¹ Department of Radio & Space Science, Chalmers University of Technology, Onsala, Sweden
¹² Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹³ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
¹⁴ CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France

¹⁵ Observatoire de Paris, LERMA UMR CNRS 8112, France
¹⁶ School of Physics and Astronomy, University of Leeds, Leeds UK
¹⁷ Centro de Astrobiología (CSIC/INTA), Laboratiorio de Astrofísica Molecular, Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹⁸ INAF Osservatorio Astrofisico di Arcetri, Florence, Italy
¹⁹ LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
²⁰ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Amsterdam, The Netherlands
²¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
²² LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France
²³ LUTH, UMR8102, Observatoire de Paris, Meudon, France
²⁴ IGN Observatorio Astronómico Nacional, Alcalá de Henares, Spain
²⁵ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
²⁶ Max-Planck-Institut für Astronomie, Heidelberg, Germany
²⁷ Departments of Physics, Astronomy and Chemistry, Ohio State University, Columbus, OH 43210, USA
²⁸ Onsala Space Observatory, Chalmers Institute of Technology, Onsala, Sweden
²⁹ SETI Institute, Mountain View, CA, USA
³⁰ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
³¹ Department of Physics and Astronomy, San Jose State University, San Jose, CA, USA
³² Department of Astronomy, Stockholm University, Stockholm, Sweden
³³ Observatoire de Paris, LUTH, and Université Denis Diderot, Meudon, France
³⁴ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George St, Toronto, ON M5S 3H8, Canada
³⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
³⁶ National University of Ireland, Maynooth, Ireland
³⁷ Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
³⁸ INAF - Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
³⁹ Institute Laboratoire d'Études du Rayonnement et de la Matière en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France and LERMA, Observatoire de Paris, Paris, France
⁴⁰ Stockholm Observatory, Stockholm, Sweden
⁴¹ Institut de RadioAstronomie Millimétrique, Grenoble, France
⁴² Department of Astronomy, University of Massachusetts, Amherst, MA, USA
⁴³ Leiden Observatory, Leiden University, Leiden, The Netherlands
⁴⁴ Kapteyn Astronomical Institute, University of Groningen, The Netherlands
⁴⁵ Department of Physics and Astronomy, University College London, London, UK

Received: 9 May 2010
Accepted: 14 June 2010

Abstract

We report the first detection of chloronium, H₂Cl⁺, in the interstellar medium, using the HIFI instrument aboard the Herschel Space Observatory. The 2₁₂–1₀₁ lines of ortho-HCl⁺ and ortho-HCl⁺ are detected in absorption towards NGC 6334I, and the 1₁₁–0₀₀ transition of para-HCl⁺ is detected in absorption towards NGC 6334I and Sgr B2(S). The H₂Cl⁺ column densities are compared to those of the chemically-related species HCl. The derived HCl/H₂Cl⁺ column density ratios, ~1–10, are within the range predicted by models of diffuse and dense photon dominated regions (PDRs). However, the observed H₂Cl⁺ column densities, in excess of 10¹³ cm^-2, are significantly higher than the model predictions. Our observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints for astrochemical models.