Letter to the Editor

Nitrogen hydrides in interstellar gas^*,**

Herschel/HIFI observations towards G10.6-0.4 (W31C)

¹ Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden e-mail: carina.persson@chalmers.se
² Centro de Astrobiologìa, CSIC-INTA, 28850 Madrid, Spain
³ Depts. of Physics, Ohio State Univ., USA
⁴ Depts. of Physics, Astronomy & Chemistry, Ohio State Univ., USA
⁵ LERMA, CNRS, Observatoire de Paris and ENS, France
⁶ California Institute of Technology, Cahill Center for Astronomy and Astrophysics 301-17, Pasadena, CA 91125, USA
⁷ Centre d'Étude Spatiale des Rayonnements (CESR), Université de Toulouse [UPS], 31062 Toulouse Cedex 9, France
⁸ CNRS/INSU, UMR 5187, 9 Avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
⁹ JPL, California Institute of Technology, Pasadena, USA
¹⁰ Nicolaus Copernicus University, Toruń, Poland
¹¹ Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
¹² The Johns Hopkins University, Baltimore, MD 21218, USA
¹³ I. Physikalisches Institut, University of Cologne, Germany
¹⁴ Institut d'Astrophysique Spatiale (IAS), Orsay, France
¹⁵ Gemini telescope, Hilo, Hawaii, USA
¹⁶ LAM, OAMP, Université Aix-Marseille & CNRS, Marseille, France
¹⁷ Laboratoire d'Astrophysique de Grenoble, France
¹⁸ Institute of Physical Chemistry, PAS, Warsaw, Poland
¹⁹ MPI für Radioastronomie, Bonn, Germany
²⁰ Dept. of Physics & Astronomy, University of Calgary, Canada
²¹ Nicolaus Copernicus Astronomical Center, Toruń, Poland
²² European Space Astronomy Centre, ESA, Madrid, Spain
²³ Institut de Radioastronomie Millimétrique, IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France
²⁴ Institute Université de Bordeaux, Laboratoire d'Astrophysique de Bordeaux, 33000 Bordeaux, France; and CNRS/INSU, UMR 5804, BP 89, 33271 Floirac Cedex, France
²⁵ Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA
²⁶ Observatorio Astronácional (IGN) and Atacama Large Millimeter/Submillimeter Array, Joint ALMA Office, Santiago, Chile

Received: 31 May 2010
Accepted: 6 July 2010

Abstract

The HIFI instrument on board the Herschel Space Observatory has been used to observe interstellar nitrogen hydrides along the sight-line towards G10.6-0.4 in order to improve our understanding of the interstellar chemistry of nitrogen. We report observations of absorption in NH N = 1 0, J = 2 1 and ortho-NH₂ 1_1,1 0_0,0. We also observed ortho-NH₃ 1₀ 0₀, and 2₀ 1₀, para-NH₃ 2₁ 1₁, and searched unsuccessfully for NH⁺. All detections show emission and absorption associated directly with the hot-core source itself as well as absorption by foreground material over a wide range of velocities. All spectra show similar, non-saturated, absorption features, which we attribute to diffuse molecular gas. Total column densities over the velocity range 11-54 km s^-1 are estimated. The similar profiles suggest fairly uniform abundances relative to hydrogen, approximately 6 × 10^-9, 3 × 10^-9, and 3 × 10^-9 for NH, NH₂, and NH₃, respectively. These abundances are discussed with reference to models of gas-phase and surface chemistry.