Nitrogen hydrides in interstellar gas

II. Analysis of Herschel/HIFI observations towards W49N and G10.6 − 0.4 (W31C) ^⋆,⋆⋆

¹ Chalmers University of TechnologyDepartment of Earth and Space Sciences, Onsala Space Observatory, 43992 Onsala, Sweden
e-mail: carina.persson@chalmers.se
² LERMA-LRA, UMR 8112 du CNRS, Observatoire de Paris, École Normale Supérieure, UPMC & UCP, 24 rue Lhomond, 75231 Paris Cedex 05, France
³ Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
⁴ Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA 22904, USA
⁵ Centro de Astrobiologìa, CSIC-INTA, 28850 Madrid, Spain
⁶ Université de Toulouse, UPS-OMP, 31028 Toulouse, France
⁷ CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁸ Department of Physics & Astronomy, Siena College, Loudonville, NY 12211, USA
⁹ Laboratoire d’Astrophysique de Grenoble, UMR 5571-CNRS, Université Joseph Fourier, 38042 Grenoble Cedex 9, France
¹⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena CA 91109, USA

Received: 20 December 2011
Accepted: 30 May 2012

Abstract

As a part of the Herschel key programme PRISMAS, we have used the Herschel/HIFI instrument to observe interstellar nitrogen hydrides along the sight-lines towards eight high-mass star-forming regions in order to elucidate the production pathways leading to nitrogen-bearing species in diffuse gas. Here, we report observations towards W49N of the NH N = 1–0, J = 2–1, and J = 1–0, ortho-NH₂N_Ka,KcJ = 1_1,13/2–0_0,01/2, ortho-NH₃J_K = 1₀–0₀ and 2₀–1₀, para-NH₃J_K = 2₁–1₁ transitions, and unsuccessful searches for NH⁺. All detections show absorption by foreground material over a wide range of velocities, as well as absorption associated directly with the hot-core source itself. As in the previously published observations towards G10.6−0.4, the NH, NH₂ and NH₃ spectra towards W49N show strikingly similar and non-saturated absorption features. We decompose the absorption of the foreground material towards W49N into different velocity components in order to investigate whether the relative abundances vary among the velocity components, and, in addition, we re-analyse the absorption lines towards G10.6−0.4 in the same manner. Abundances, with respect to molecular hydrogen, in each velocity component are estimated using CH, which is found to correlate with H₂ in the solar neighbourhood diffuse gas. The analysis points to a co-existence of the nitrogen hydrides in diffuse or translucent interstellar gas with a high molecular fraction. Towards both sources, we find that NH is always at least as abundant as both o-NH₂ and o-NH₃, in sharp contrast to previous results for dark clouds. We find relatively constant N(NH)/N(o-NH₃) and N(o-NH₂)/N(o-NH₃) ratios with mean values of 3.2 and 1.9 towards W49N, and 5.4 and 2.2 towards G10.6−0.4, respectively. The mean abundance of o-NH₃ is  ~2  ×  10^-9 towards both sources. The nitrogen hydrides also show linear correlations with CN and HNC towards both sources, and looser correlations with CH. The upper limits on the NH⁺ abundance indicate column densities  ≲2–14% ofN(NH), which is in contrast to the behaviour of the abundances of CH⁺ and OH⁺ relative to the values determined for the corresponding neutrals CH and OH. Surprisingly low values of the ammonia ortho-to-para ratio are found in both sources,  ≈ 0.5–0.7  ±  0.1, in the strongest absorption components. This result cannot be explained by current models as we had expected to find a value of unity or higher.