Letter to the Editor

First detection of ND in the solar-mass protostar IRAS16293-2422^*

¹ Laboratoire d'Astrophysique de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, Grenoble, France e-mail: aurore.bacmann@obs.ujf-grenoble.fr
² Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, Floirac, France
³ CNRS/INSU, UMR 5804, Floirac Cedex, France
⁴ Centre d'Étude Spatiale des Rayonnements, Université Paul Sabatier, Toulouse, France
⁵ CNRS/INSU, UMR 5187, Toulouse, France
⁶ Max-Planck-Institut für Radioastronomie, Bonn, Germany
⁷ LERMA and UMR 8112 du CNRS, Observatoire de Paris, Paris, France
⁸ Centro de Astrobiología, CSIC-INTA, Madrid, Spain
⁹ Max-Planck-Institut für Astronomie, Heidelberg, Germany
¹⁰ Department of Astronomy, University of Michigan, Ann Arbor, USA
¹¹ California Institute of Technology, Pasadena, USA
¹² Astronomical Institute Anton Pannekoek, University of Amsterdam, Amsterdam, The Netherlands
¹³ Department of Astrophysics/IMAPP, Radboud University Nijmegen, Nijmegen, The Netherlands
¹⁴ Laboratoire d'Études du Rayonnement et de la Matière en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France
¹⁵ Harvard-Smithsonian Center for Astrophysics, Cambridge MA, USA
¹⁶ Physikalisches Institut, Universität zu Köln, Köln, Germany
¹⁷ INAF - Istituto di Fisica dello Spazio Interplanetario, Roma, Italy
¹⁸ Infrared Processing and Analysis Center, Caltech, Pasadena, USA
¹⁹ School of Physics and Astronomy, University of Leeds, Leeds, UK
²⁰ INAF Osservatorio Astrofisico di Arcetri, Florence, Italy
²¹ IGN Observatorio Astronómico Nacional, Alcalá de Henares, Spain
²² Jet Propulsion Laboratory, Caltech, Pasadena, CA 91109, USA
²³ SRON Netherlands Institute for Space Research, Groningen, The Netherlands
²⁴ Ohio State University, Columbus, OH, USA
²⁵ Johns Hopkins University, Baltimore MD, USA
²⁶ INAF - Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
²⁷ Institut de RadioAstronomie Millimétrique, Grenoble, France
²⁸ Leiden Observatory, Leiden University, Leiden, The Netherlands
²⁹ Kapteyn Astronomical Institute, University of Groningen, The Netherlands
³⁰ Department of Physics and Astronomy, University College London, London, UK
³¹ Institute of Astronomy, ETH Zürich, Zürich, Switzerland

Received: 31 May 2010
Accepted: 30 June 2010

Abstract

Context. In the past decade, much progress has been made in characterising the processes leading to the enhanced deuterium fractionation observed in the ISM and in particular in the cold, dense parts of star forming regions such as protostellar envelopes. Very high molecular D/H ratios have been found for saturated molecules and ions. However, little is known about the deuterium fractionation in radicals, even though simple radicals often represent an intermediate stage in the formation of more complex, saturated molecules. The imidogen radical NH is such an intermediate species for the ammonia synthesis in the gas phase. Many of these light molecules however have their fundamental transitions in the submillimetre domain and their detection is hampered by the opacity of the atmosphere at these wavelengths. Herschel/HIFI represents a unique opportunity to study the deuteration and formation mechanisms of species not observable from the ground.

Aims. We searched here for the deuterated radical ND in order to determine the deuterium fractionation of imidogen and constrain the deuteration mechanism of this species.

Methods. We observed the solar-mass Class 0 protostar IRAS16293-2422 with the heterodyne instrument HIFI in Bands 1a (480–560 GHz), 3b (858–961 GHz), and 4a (949–1061 GHz) as part of the Herschel key programme CHESS (Chemical HErschel Survey of Star forming regions).

Results. The deuterated form of the imidogen radical ND was detected and securely identified with 2 hyperfine component groups of its fundamental transition (N = 0–1) at 522.1 and 546.2 GHz, in absorption against the continuum background emitted from the nascent protostar. The 3 groups of hyperfine components of its hydrogenated counterpart NH were also detected in absorption. The absorption arises from the cold envelope, where many deuterated species have been shown to be abundant. The estimated column densities are ~2 × 10¹⁴ cm^-2 for NH and ~ 1.3 × 10¹⁴ cm^-2 for ND. We derive a very high deuterium fractionation with an [ND]/[NH] ratio of between 30 and 100%.

Conclusions. The deuterium fractionation of imidogen is of the same order of magnitude as that in other molecules, which suggests that an efficient deuterium fractionation mechanism is at play. We discuss two possible formation pathways for ND, by means of either the reaction of N⁺ with HD, or deuteron/proton exchange with NH.