Letter to the Editor

Detection of interstellar oxidaniumyl: Abundant H₂O⁺ towards the star-forming regions DR21, Sgr B2, and NGC6334^*

¹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany e-mail: ossk@pk1.uni-koeln.de
² SRON Netherlands Institute for Space Research, P.O. Box 800, 9700 AV Groningen, The Netherlands
³ California Institute of Technology, Pasadena, CA 91125 USA
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany
⁵ Institute of Astronomy, ETH Zürich, 8093 Zürich, Switzerland
⁶ University of Michigan, Ann Arbor, MI 48197 USA
⁷ Laboratoire d'Astrophysique de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, Grenoble, France
⁸ Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, France; CNRS/INSU, UMR 5804, Floirac, France
⁹ Istituto Fisica Spazio Interplanetario INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁰ Leiden Observatory, Universiteit Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ Université de Toulouse, UPS, CESR, 9 avenue du colonel Roche, 31062 Toulouse Cedex 4, France
¹² Institut d'Astrophysique Spatiale, Université Paris-Sud, Bât. 121, 91405 Orsay Cedex, France
¹³ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61, Av. de l'Observatoire, 75014 Paris, France
¹⁴ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT UK
¹⁵ CNRS, UMR 5187, 31028 Toulouse, France
¹⁶ Centro de Astrobiología, CSIC-INTA, 28850, Madrid, Spain
¹⁷ NAF Osservatorio Astrofisico di Arcetri, Florence Italy
¹⁸ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Amsterdam, The Netherlands
¹⁹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
²⁰ Université Pierre et Marie Curie, LPMAA UMR CNRS 7092, Case 76, 4 place Jussieu, 75252 Paris Cedex 05, France
²¹ LERMA & UMR 8112 du CNRS, Observatoire de Paris and École Normale Supérieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
²² Department of Astronomy and Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
²³ Observatorio Astronómico Nacional, Apdo. 112, 28803 Alcalá de Henares, Spain
²⁴ Jet Propulsion Laboratory, 4800 Oak Grove Drive, MC 302-231, Pasadena, CA 91109, USA
²⁵ Astronomy Department, University of Maryland, College Park, MD 20742, USA
²⁶ Ohio State University, Columbus, OH 43210, USA
²⁷ NRC/HIA Victoria, BC V9E 2E7, Canada
²⁸ Instituto de Radio Astronomía Milimétrica (IRAM), Avenida Divina Pastora 7, Local 20, 18012 Granada, Spain
²⁹ Department of Physics and Astronomy, University of Waterloo, Waterloo, ON N2L 3G1, Canada
³⁰ University of Western Ontario, Department of Physics & Astronomy, London, N6A 3K7 Ontario, Canada
³¹ Institut für 4D-Technologien, FHNW, 5210 Windisch, Switzerland
³² Center for Astrophysics, Cambridge MA 02138, USA
³³ Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Mumbai 400005, India
³⁴ Experimental Physics Dept., National University of Ireland Maynooth, Co. Kildare, Ireland
³⁵ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³⁶ INAF - Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
³⁷ Centre for Radio Astronomy, University of Calgary, Canada
³⁸ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
³⁹ European Space Astronomy Centre, Urb. Villafranca del Castillo, PO Box 50727, Madrid 28080, Spain
⁴⁰ Department of Physics and Astronomy, University College London, London, UK
⁴¹ Atacama Large Millimeter Array, Joint ALMA Office, Santiago, Chile

Received: 30 March 2010
Accepted: 7 May 2010

Abstract

Aims. We identify a prominent absorption feature at 1115 GHz, detected in first HIFI spectra towards high-mass star-forming regions, and interpret its astrophysical origin.

Methods. The characteristic hyperfine pattern of the H₂O⁺ ground-state rotational transition, and the lack of other known low-energy transitions in this frequency range, identifies the feature as H₂O⁺ absorption against the dust continuum background and allows us to derive the velocity profile of the absorbing gas. By comparing this velocity profile with velocity profiles of other tracers in the DR21 star-forming region, we constrain the frequency of the transition and the conditions for its formation.

Results. In DR21, the velocity distribution of H₂O⁺ matches that of the [C ii] line at 158 μm and of OH cm-wave absorption, both stemming from the hot and dense clump surfaces facing the H ii-region and dynamically affected by the blister outflow. Diffuse foreground gas dominates the absorption towards Sgr B2. The integrated intensity of the absorption line allows us to derive lower limits to the H₂O⁺ column density of 7.2 × 10¹² cm^-2 in NGC 6334, 2.3 × 10¹³ cm^-2 in DR21, and 1.1 × 10¹⁵ cm^-2 in Sgr B2.