Letter to the Editor

Nitrogen hydrides in the cold envelope of IRAS 16293-2422 ^*,**

¹ Laboratoire d'Astrophysique de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, Grenoble, France
² Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, CNRS/INSU, UMR 5804, Floirac, France
³ Centre d'Étude Spatiale des Rayonnements, Université Paul Sabatier, Toulouse 3, CNRS UMR 5187, Toulouse, France
⁴ Max-Planck-Institut für Radioastronomie, Bonn, Germany
⁵ Department of Astronomy, University of Michigan, Ann Arbor, USA
⁶ California Institute of Technology, Pasadena, USA
⁷ Centro de Astrobiología, CSIC-INTA, Madrid, Spain
⁸ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Amsterdam, The Netherlands
⁹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
¹⁰ LERMA, UMR 8112-CNRS, Ecole Normale Supérieure et Observatoire de Paris, France
¹¹ Max-Planck-Institut für Astronomie, Heidelberg, Germany
¹² Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA
¹³ LERMA and UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
¹⁴ Physikalisches Institut, Universität zu Köln, Köln, Germany
¹⁵ INAF – Istituto di Fisica dello Spazio Interplanetario, Roma, Italy
¹⁶ Infared Processing and Analysis Center, Caltech, Pasadena, USA
¹⁷ School of Physics and Astronomy, University of Leeds, Leeds UK
¹⁸ INAF Osservatorio Astrofisico di Arcetri, Florence Italy
¹⁹ IGN Observatorio Astronómico Nacional, Alcalá de Henares, Spain
²⁰ Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA
²¹ SRON Netherlands Institute for Space Research, Groningen, The Netherlands
²² Ohio State University, Columbus, OH, USA
²³ Johns Hopkins University, Baltimore MD, USA
²⁴ INAF – Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
²⁵ Institut de RadioAstronomie Millimétrique, Grenoble, France
²⁶ Leiden Observatory, Leiden University, Leiden, The Netherlands
²⁷ Kapteyn Astronomical Institute, University of Groningen, The Netherlands
²⁸ Department of Physics and Astronomy, University College London, London, UK

Received: 22 June 2010
Accepted: 2 September 2010

Abstract

Nitrogen is the fifth most abundant element in the Universe, yet the gas-phase chemistry of N-bearing species remains poorly understood. Nitrogen hydrides are key molecules of nitrogen chemistry. Their abundance ratios place strong constraints on the production pathways and reaction rates of nitrogen-bearing molecules. We observed the class 0 protostar IRAS 16293-2422 with the heterodyne instrument HIFI, covering most of the frequency range from 0.48 to 1.78 THz at high spectral resolution. The hyperfine structure of the amidogen radical o-NH₂ is resolved and seen in absorption against the continuum of the protostar. Several transitions of ammonia from 1.2 to 1.8 THz are also seen in absorption. These lines trace the low-density envelope of the protostar. Column densities and abundances are estimated for each hydride. We find that NH:NH₂:NH₃ ≈ 5:1:300. Dark clouds chemical models predict steady-state abundances of NH₂ and NH₃ in reasonable agreement with the present observations, whilst that of NH is underpredicted by more than one order of magnitude, even using updated kinetic rates. Additional modelling of the nitrogen gas-phase chemistry in dark-cloud conditions is necessary before having recourse to heterogen processes.