Letter to the Editor

First hyperfine resolved far-infrared OH spectrum from a star-forming region^⋆,⋆⋆

¹ Institute for Astronomy, ETH Zurich, 8093 Zurich, Switzerland
e-mail: wsusanne@astro.phys.ethz.ch
² Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, 33271 Floirac Cedex, France
⁵ CNRS, UMR 5804, Laboratoire d’Astrophysique de Bordeaux, 2 rue de l’Observatoire, BP 89, 33271 Floirac Cedex, France
⁶ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
⁷ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands
⁸ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV, Groningen, The Netherlands
⁹ Departamento de Astrofísica, Centro de Astrobiología (CSIC-INTA), 28850, Madrid, Spain
¹⁰ Department of Physics and Astronomy, Denison University, Granville, OH, 43023, USA

Received: 15 March 2011
Accepted: 25 May 2011

Abstract

OH is an important molecule in the H₂O chemistry and the cooling budget of star-forming regions. The goal of the Herschel key program “Water In Star-forming regions with Herschel” (WISH) is to study H₂O and related species during protostellar evolution. Our aim in this Letter is to assess the origin of the OH emission from star-forming regions and constrain the properties of the emitting gas. High-resolution observations of the OH  triplet at 1837.8   GHz (163.1   μm) towards the high-mass star-forming region W3 IRS 5 with the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel reveal the first hyperfine velocity-resolved OH far-infrared spectrum of a star-forming region. The line profile of the OH emission shows two components: a narrow component (FWHM  ≈  4−5   km   s^-1) with partially resolved hyperfine structure resides on top of a broad (FWHM  ≈  30   km   s^-1) component. The narrow emission agrees well with results from radiative transfer calculations of a spherical envelope model for W3 IRS 5 with a constant OH abundance of x_OH ≈ 8  ×  10^-9. Comparison with H₂O yields OH/H₂O abundance ratios of around 10^-3 for T ≳ 100   K and around unity for T ≲ 100   K, consistent with the current picture of the dense cloud chemistry with freeze-out and photodesorption. The broad component is attributed to outflow emission. An abundance ratio of OH/H₂O  ≳  0.028 in the outflow is derived from comparison with results of water line modeling. This ratio can be explained by a fast J-type shock or a slower UV-irradiated C-type shock.