Nitrogen hydrides in interstellar gas II. Analysis of Herschel/HIFI observations towards W49N and G10.6-0.4 (W31C)
Artikel i vetenskaplig tidskrift, 2012

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-NH2 N_(Ka, K_c) J = 1_(1,1) 3/2 - 0_(0,0) 1/2, ortho-NH3 J_K = 1_0 - 0_0 and 2_0 - 1_0, para-NH3 J_K = 2_1 - 1_1 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-NH4 is ~2x10^-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 % of N(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.

ISM: molecules

ISM: abundances

Astrochemistry

Molecular processes

Sub-millimetre: ISM

Line: formation

Författare

Carina Persson

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

Massimo De Luca

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

B. Mookerjea

Tata Institute of Fundamental Research

Henrik Olofsson

Chalmers, Rymd- och geovetenskap, Onsala rymdobservatorium

John H Black

Chalmers, Rymd- och geovetenskap, Radioastronomi och astrofysik

M. Gerin

LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres

E. Herbst

University of Virginia

Tom A. Bell

Centro de Astrobiologia (CAB)

A. Coutens

Centre national de la recherche scientifique (CNRS)

Université de Toulouse

B. Godard

Centro de Astrobiologia (CAB)

J.R. Goicoechea

Centro de Astrobiologia (CAB)

G.E. Hassel

Siena College

P. Hily-Blant

Université Grenoble Alpes

K.M. Menten

Max Planck-institutet

Holger Muller

Universität zu Köln

T. J. Pearson

Jet Propulsion Laboratory, California Institute of Technology

S. Yu

Jet Propulsion Laboratory, California Institute of Technology

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 543 Article Number: A145 145-179 A145

Ämneskategorier

Astronomi, astrofysik och kosmologi

Fundament

Grundläggande vetenskaper

Infrastruktur

Onsala rymdobservatorium

DOI

10.1051/0004-6361/201118686