Letter to the Editor

Ortho-to-para ratio of interstellar heavy water^*

¹ Centre d'Étude Spatiale des Rayonnements, Université Paul Sabatier, Toulouse, France e-mail: vastel@cesr.fr
² CNRS/INSU, UMR 5187, Toulouse, France

³ Laboratoire d'Astrophysique de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, Grenoble, France

⁴ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, Floirac, France

⁵ CNRS/INSU, UMR 5804, Floirac Cedex, France

⁶ Centro de Astrobiologìa, CSIC-INTA, Madrid, Spain

⁷ Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 5209-CNRS, Dijon Cedex, France 

⁸ Department of Astronomy, University of Michigan, Ann Arbor, USA

⁹ California Institute of Technology, Pasadena, USA

¹⁰ Astronomical Institute Anton Pannekoek, University of Amsterdam, Amsterdam, The Netherlands

¹¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands

¹² Laboratoire d'Études du Rayonnement et de la Matière en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France

¹³ Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA

¹⁴ Max-Planck-Institut für Radioastronomie, Bonn, Germany

¹⁵ Physikalisches Institut, Universität zu Köln, Köln, Germany

¹⁶ INAF - Istituto di Fisica dello Spazio Interplanetario, Roma, Italy

¹⁷ Infared Processing and Analysis Center, Caltech, Pasadena, USA

¹⁸ School of Physics and Astronomy, University of Leeds, Leeds, UK

¹⁹ INAF Osservatorio Astrofisico di Arcetri, Florence, Italy

²⁰ IGN Observatorio Astronómico Nacional, Alcalá de Henares, Spain
²¹ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA

²² SRON Netherlands Institute for Space Research, Groningen, The Netherlands

²³ Max-Planck-Institut für Astronomie, Heidelberg, Germany

²⁴ Ohio State University, Columbus, OH, USA

²⁵ Johns Hopkins University, Baltimore MD, USA

²⁶ INAF - Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy

²⁷ Institut de RadioAstronomie Millimétrique, Grenoble, France

²⁸ Leiden Observatory, Leiden University, Leiden, The Netherlands

²⁹ Kapteyn Astronomical Institute, University of Groningen, The Netherlands

³⁰ Department of Physics and Astronomy, University College London, London, UK

³¹ Chalmers University of Technology, Gøterborg, Sweden


Received: 28 May 2010
Accepted: 25 July 2010

Abstract

Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular D/H ratios have been found in the environments of low-mass star-forming regions, and in particular the Class 0 protostar IRAS  16293-2422.

Aims. The CHESS (Chemical HErschel Surveys of Star forming regions) key program aims to study the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI instrument provide a unique opportunity to observe the fundamental 1- transition of the ortho-D₂O molecule, which is inaccessible from the ground, and determine the ortho-to-para D₂O ratio.

Methods. We detected the fundamental transition of the ortho-D₂O molecule at 607.35 GHz towards IRAS  16293-2422. The line is seen in absorption with a line opacity of 0.62 ± 0.11 (1σ). From the previous ground-based observations of the fundamental 1 – 1 transition of para-D₂O seen in absorption at 316.80 GHz, we estimate a line opacity of 0.26 ± 0.05 (1σ).

Results. We show that the observed absorption is caused by the cold gas in the envelope of the protostar. Using these new observations, we estimate for the first time the ortho-to-para D₂O ratio to be lower than 2.6 at a 3σ level of uncertainty, which should be compared with the thermal equilibrium value of 2:1.