Letter to the Editor

Herschel observations of ortho- and para-oxidaniumyl (H₂O⁺) in spiral arm clouds toward Sagittarius B2(M) ^*,**

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail: schilke@ph1.uni-koeln.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁴ Departments of Physics, Astronomy and Chemistry, Ohio State University, Columbus, OH 43210, USA
⁵ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA

⁶ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁷ California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
⁸ LERMA & UMR8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
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 LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
¹⁴ LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France
¹⁵ LUTH, UMR8102, Observatoire de Paris, Meudon, France
¹⁶ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA

¹⁷ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁸ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹⁹ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George St, Toronto, ON M5S 3H8, Canada
²⁰ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
²¹ National University of Ireland Maynooth. Ireland
²² SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
²³ Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
²⁴ University of Massachusetts, Astronomy Dept., 710 N. Pleasant St., LGRT-619E, Amherst, MA 01003-9305, USA

Received: 30 May 2010
Accepted: 28 June 2010

Abstract

H₂O⁺ has been observed in its ortho- and para- states toward the massive star forming core Sgr B2(M), located close to the Galactic center. The observations show absorption in all spiral arm clouds between the Sun and Sgr B2. The average o/p ratio of H₂O⁺ in most velocity intervals is 4.8, which corresponds to a nuclear spin temperature of 21 K. The relationship of this spin temperature to the formation temperature and current physical temperature of the gas hosting H₂O⁺ is discussed, but no firm conclusion is reached. In the velocity interval 0–60 km s^-1, an ortho/para ratio of below unity is found, but if this is due to an artifact of contamination by other species or real is not clear.