Letter to the Editor

Water abundances in high-mass protostellar envelopes: Herschel observations with HIFI ^*,**

¹ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands e-mail: M.Marseille@sron.nl
² Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
³ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, Floirac, France
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Centro de Astrobiología. Departamento de Astrofísica. CSIC-INTA. Carretera de Ajalvir, Km 4, Torrejón de Ardoz. 28850, Madrid, Spain
⁶ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
⁹ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII,3. 28014 Madrid, Spain
¹⁰ INAF - Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹¹ Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
¹² Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
¹³ Department of Radio and Space Science, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁴ California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
¹⁵ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹⁶ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
¹⁷ CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
¹⁸ Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
¹⁹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, 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 Observatorio Astronómico Nacional, Apartado 112, 28803 Alcalá de Henares, Spain
²⁴ Jet Propulsion Laboratory, 4800 Oak Grove Drive, MC 302-306, Pasadena, CA 91109 USA
²⁵ INAF - Osservatorio Astronomico di Roma, 00040 Monte Porzio catone, Italy
²⁶ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
²⁷ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
²⁸ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K., Denmark
²⁹ Department of Astronomy, Stockholm University, Albania, 106 91 Stockholm, Sweden
³⁰ California Institute of Technology, Cahill Center for Astronomy and Astrophysics, MS 301-17, Pasadena, CA 91125, USA
³¹ The University of Western Ontario, Department of Physics and Astronomy, London, Ontario, N6A 3K7, Canada
³² Microwave Laboratory, ETH Zurich, 8092 Zurich, Switzerland
³³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA
³⁴ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³⁵ Department of Physics and Astronomy, University of Calgary, Calgary, T2N 1N4, AB, Canada
³⁶ Instituto de Radioastronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Núcleo Central, 18012 Granada, Spain
³⁷ KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 31 May 2010
Accepted: 15 July 2010

Abstract

Aims. We derive the dense core structure and the water abundance in four massive star-forming regions in the hope of understanding the earliest stages of massive star formation.

Methods. We present Herschel/HIFI observations of the para-H₂O 1₁₁–0₀₀ and 2₀₂–1₁₁ and the para-H₂¹⁸O 1₁₁–0₀₀ transitions. The envelope contribution to the line profiles is separated from contributions by outflows and foreground clouds. The envelope contribution is modeled with Monte-Carlo radiative transfer codes for dust and molecular lines (MC3D and RATRAN), and the water abundance and the turbulent velocity width as free parameters.

Results. While the outflows are mostly seen in emission in high-J lines, envelopes are seen in absorption in ground-state lines, which are almost saturated. The derived water abundances range from 5×10^-10 to 4×10^-8 in the outer envelopes. We detect cold clouds surrounding the protostar envelope, thanks to the very high quality of the Herschel/HIFI data and the unique ability of water to probe them. Several foreground clouds are also detected along the line of sight.

Conclusions. The low H₂O abundances in massive dense cores are in accordance with the expectation that high densities and low temperatures lead to freeze-out of water on dust grains. The spread in abundance values is not clearly linked to physical properties of the sources.