Letter to the Editor

Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C^*

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 Departments of Physics, Astronomy and Chemistry, Ohio State University, Columbus, OH 43210, USA
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁷ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁸ LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
⁹ Chalmers University of Technology, Göteborg, Sweden
¹⁰ Institut d'Astrophysique Spatiale (IAS), Orsay, France

¹¹ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
¹² CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France

¹³ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
¹⁴ Centro de Astrobiología (CSIC/INTA), Laboratiorio de Astrofísica Molecular, Ctra. de Torrejón a Ajalvir, km 4 28850, Torrejón de Ardoz, Madrid, Spain
¹⁵ LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France
¹⁶ LUTH, UMR8102, Observatoire de Paris, Meudon, France
¹⁷ Gemini Telescope, Hilo, Hawaii, USA
¹⁸ Institute of Physical Chemistry, PAS, Warsaw, Poland
¹⁹ Nicolaus Copernicus University, Torun, Poland
²⁰ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
²¹ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
²² Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George St, Toronto, ON M5S 3H8, Canada
²³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
²⁴ Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
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 Nicolaus Copernicus Astronomical Center, Poland
²⁹ European Space Astronomy Centre, ESA, Madrid, Spain
³⁰ University of Massachusetts, Astronomy Dept., 710 N. Pleasant St., LGRT-619E, Amherst, MA 01003-9305, USA

Received: 28 May 2010
Accepted: 18 June 2010

Abstract

We present Herschel/HIFI observations of the fundamental rotational transitions of ortho- and para-H₂¹⁶O and H₂¹⁸O in absorption towards Sagittarius B2(M) and W31C. The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3, within the observational uncertainties. However, somewhat unexpectedly, we derive a low ortho/para ratio of 2.35 ± 0.35, corresponding to a spin temperature of ~27 K, towards Sagittarius B2(M) at velocities of the expanding molecular ring. Water molecules in this region appear to have formed with, or relaxed to, an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust.