Letter to the Editor

Herschel observations of EXtra-Ordinary Sources (HEXOS): Observations of H₂O and its isotopologues towards Orion KL ^*,**

¹ Harvard-Smithsonian Center for Astrophysics (CfA), 60 Garden Street, Mail Stop 66, Cambridge MA 02138, USA e-mail: gmelnick@cfa.harvard.edu
² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³ Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
⁴ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125 USA
⁵ California Institute of Technology, Division of Geological and Planetary Sciences, MS 150-21, Pasadena, CA 91125, USA
⁶ LERMA & UMR8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France

⁷ Centre d'Étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
⁸ CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
⁹ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble, Cedex 9, France.
¹⁰ Centro de Astrobiología (CSIC/INTA), Laboratiorio de Astrofísica Molecular, Ctra. de Torrejón a Ajalvir, km 4 28850, Torrejón de Ardoz, Madrid, Spain
¹¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹² LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
¹³ LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France
¹⁴ LUTH, UMR8102, Observatoire de Paris, Meudon, France
¹⁵ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹⁶ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA

¹⁷ Departments of Physics, Astronomy and Chemistry, Ohio State University, Columbus, OH 43210, USA
¹⁸ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁹ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125
²⁰ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George St, Toronto, ON M5S 3H8, Canada
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

Received: 30 May 2010
Accepted: 12 July 2010

Abstract

We report the detection of more than 48 velocity-resolved ground rotational state transitions of H₂¹⁶O, H₂¹⁸O, and H₂¹⁷O – most for the first time – in both emission and absorption toward Orion KL using Herschel/HIFI. We show that a simple fit, constrained to match the known emission and absorption components along the line of sight, is in excellent agreement with the spectral profiles of all the water lines. Using the measured H₂¹⁸O line fluxes, which are less affected by line opacity than their H₂¹⁶O counterparts, and an escape probability method, the column densities of H₂¹⁸O associated with each emission component are derived. We infer total water abundances of 7.4 × 10^-5, 1.0 × 10^-5, and 1.6 × 10^-5 for the plateau, hot core, and extended warm gas, respectively. In the case of the plateau, this value is consistent with previous measures of the Orion-KL water abundance as well as those of other molecular outflows. In the case of the hot core and extended warm gas, these values are somewhat higher than water abundances derived for other quiescent clouds, suggesting that these regions are likely experiencing enhanced water-ice sublimation from (and reduced freeze-out onto) grain surfaces due to the warmer dust in these sources.