Letter to the Editor

Water abundance variations around high-mass protostars: HIFI observations of the DR21 region^*

¹ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands e-mail: vdtak@sron.nl
² Kapteyn Institute, University of Groningen, The Netherlands
³ Laboratoire d'Astrophysique de Bordeaux, Floirac, France
⁴ Max-Planck-Institut für Radioastronomie, Bonn, Germany
⁵ Denison University, Granville OH, USA
⁶ Harvard-Smithsonian Center for Astrophysics, Cambridge, USA
⁷ Sterrewacht, Universiteit Leiden, The Netherlands
⁸ MPI für Extraterrestrische Physik, Garching, Germany
⁹ Institute of Astronomy, ETH Zürich, 8093 Zürich, Switzerland
¹⁰ School of Physics and Astronomy, University of Leeds, UK
¹¹ Herzberg Institute of Astrophysics, Victoria, Canada
¹² Dept. of Physics and Astronomy, University of Victoria, Canada
¹³ Chalmers University of Technology, 41296 Göteborg, Sweden
¹⁴ Observatorio Astronómico Nacional, Alcalá de Henares, Spain
¹⁵ INAF – Istituto di Fisica dello Spazio Interplanetario, Roma, Italy
¹⁶ Dept. of Astronomy, University of Michigan, Ann Arbor, USA
¹⁷ California Institute of Technology, Pasadena CA 91125, USA
¹⁸ CAB, INTA-CSIC, Torrejón de Ardoz, Spain
¹⁹ Observatoire de Paris-Meudon, Meudon, France
²⁰ University of Amsterdam, The Netherlands
²¹ LERMA and UMR 8112 du CNRS, Observatoire de Paris, France
²² Dept. of Physics and Astronomy, University of Waterloo, Canada
²³ Joint ALMA Observatory, Santiago, Chile
²⁴ Centre for Star and Planet Formation, U. of Copenhagen, Denmark
²⁵ Department of Astronomy, Stockholm University, Sweden
²⁶ Johns Hopkins University, Baltimore, USA
²⁷ KOSMA, I. Physik. Institut, Universität zu Köln, Germany
²⁸ JPL, California Institute of Technology, Pasadena, CA 91109, USA
²⁹ Dept. of Physics and Astronomy, University of Calgary, Canada
³⁰ CESR, Université de Toulouse, France
³¹ European Space Astronomy Centre, ESA, Madrid, Spain

Received: 26 March 2010
Accepted: 20 April 2010

Abstract

Context. Water is a key molecule in the star formation process, but its spatial distribution in star-forming regions is not well known.

Aims. We study the distribution of dust continuum and H₂O and ¹³CO line emission in DR21, a luminous star-forming region with a powerful outflow and a compact H ii region.

Methods. Herschel-HIFI spectra near 1100 GHz show narrow ¹³CO 10–9 emission and H₂O 1₁₁–0₀₀ absorption from the dense core and broad emission from the outflow in both lines. The H₂O line also shows absorption by a foreground cloud known from ground-based observations of low-J CO lines.

Results. The dust continuum emission is extended over 36” FWHM, while the ¹³CO and H₂O lines are confined to ≈24” or less. The foreground absorption appears to peak further North than the other components. Radiative transfer models indicate very low abundances of ~2×10^-10 for H₂O and ~8×10^-7 for ¹³CO in the dense core, and higher H₂O abundances of ~4×10^-9 in the foreground cloud and ~7×10^-7 in the outflow.

Conclusions. The high H₂O abundance in the warm outflow is probably due to the evaporation of water-rich icy grain mantles, while the H₂O abundance is kept down by freeze-out in the dense core and by photodissociation in the foreground cloud.