Letter to the Editor

Herschel observations of EXtra-Ordinary Sources (HEXOS): Detection of hydrogen fluoride in absorption towards Orion KL^*

¹ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA e-mail: tgp@submm.caltech.edu
² Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
³ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁴ Centre d'étude Spatiale des Rayonnements, Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
⁵ CNRS/INSU, UMR 5187, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
⁶ Laboratoire d'Astrophysique de l'Observatoire de Grenoble, BP 53, 38041 Grenoble, Cedex 9, France
⁷ Centro de Astrobiología (CSIC/INTA), Laboratiorio de Astrofísica Molecular, Ctra. de Torrejón a Ajalvir, km 4 28850, Torrejón de Ardoz, Madrid, Spain
⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ LERMA, CNRS UMR8112, Observatoire de Paris and École Normale Supérieure, 24 Rue Lhomond, 75231 Paris Cedex 05, France
¹⁰ LPMAA, UMR7092, Université Pierre et Marie Curie, Paris, France
¹¹ LUTH, UMR8102, Observatoire de Paris, Meudon, France
¹² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹³ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA
¹⁴ Departments of Physics, Astronomy and Chemistry, Ohio State University, Columbus, OH 43210, USA
¹⁵ National Research Council Canada, Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁶ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
¹⁷ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St George St, Toronto, ON M5S 3H8, Canada
¹⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
¹⁹ National University of Ireland Maynooth, Ireland
²⁰ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
²¹ Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada
²² MPI für Sonnensystemforschung, 37191 Katlenburg-Lindau, Germany

Received: 30 March 2010
Accepted: 20 April 2010

Abstract

We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal of contaminating features associated with common molecules (“weeds”), the HF spectrum shows a P-Cygni profile, with weak redshifted emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 × 10¹³ cm^-2 for the HF column density responsible for the broad absorption component. Using our best estimate of the H₂ column density within the low-velocity molecular outflow, we obtain a lower limit of ~1.6 × 10^-10 for the HF abundance relative to hydrogen nuclei, corresponding to ~0.6% of the solar abundance of fluorine. This value is close to that inferred from previous ISO observations of HF J = 2–1 absorption towards Sgr B2, but is in sharp contrast to the lower limit of 6 × 10^-9 derived by Neufeld et al. for cold, foreground clouds on the line of sight towards G10.6-0.4.