Letter to the Editor

Variations in H₂O⁺/H₂O ratios toward massive star-forming regions^*

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: wyrowski@mpifr-bonn.mpg.de
² SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
⁴ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, Floirac, France
⁵ Max Planck Institut for Extraterestrische Physik, Garching, Germany
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
⁸ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
⁹ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁰ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
¹¹ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹² IGN Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain
¹³ INAF - Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁴ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹⁵ California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
¹⁶ Department of Astrophysics, CAB, INTA-CSIC, Crta Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Spain
¹⁷ Observatoire de Paris-Meudon, LERMA UMR CNRS 8112, 5 place Jules Janssen, 92195 Meudon Cedex, France
¹⁸ Department of Molecular and Infrared Astrophysics, Consejo Superior de Investigaciones Cientificas, C/ Serrano 121, 28006 Madrid, Spain
¹⁹ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
²⁰ Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA
²¹ LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
²² University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada
²³ Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile
²⁴ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark
²⁵ Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
²⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
²⁷ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²⁸ Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada
²⁹ Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 31 May 2010
Accepted: 19 July 2010

Abstract

Early results from the Herschel Space Observatory revealed the water cation H₂O⁺ to be an abundant ingredient of the interstellar medium. Here we present new observations of the H₂O and H₂O⁺ lines at 1113.3 and 1115.2 GHz using the Herschel Space Observatory toward a sample of high-mass star-forming regions to observationally study the relation between H₂O and H₂O⁺. Nine out of ten sources show absorption from H₂O⁺ in a range of environments: the molecular clumps surrounding the forming and newly formed massive stars, bright high-velocity outflows associated with the massive protostars, and unrelated low-density clouds along the line of sight. Column densities per velocity component of H₂O⁺ are found in the range of 10¹² to a few 10¹³ cm^-2. The highest N(H₂O⁺) column densities are found in the outflows of the sources. The ratios of H₂O⁺/H₂O are determined in a range from 0.01 to a few and are found to differ strongly between the observed environments with much lower ratios in the massive (proto)cluster envelopes (0.01-0.1) than in outflows and diffuse clouds. Remarkably, even for source components detected in H₂O in emission, H₂O⁺ is still seen in absorption.