Herschel/HIFI observations of the circumstellar ammonia lines in IRC+10216 ^⋆,⋆⋆

¹ N. Copernicus Astronomical Center, Rabiańska 8, 87-100 Toruń, Poland
e-mail: schmidt@ncac.torun.pl
² Key Laboratory for the Structure and Evolution of Celestial Objects, Yunnan Observatories, Chinese Academy of Sciences, PO Box 110, Kunming, Yunnan Province, PR China
³ Observatorio Astronómico Nacional. Ap 112, 28803 Alcalá de Henares, Spain
⁴ Observatorio Astronómico Nacional (IGN), Alfonso XII N ◦ 3, 28014 Madrid, Spain
⁵ ICMM, CSIC, group of Molecular Astrophysics, C/Sor Juana Inés de la Cruz N3, 28049 Cantoblanco, Madrid, Spain
⁶ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁷ Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
⁸ Chalmers University of Technology, Department of Earth and Space Sciences, Onsala Space Observatory, 439 92 Onsala, Sweden
⁹ European Space Astronomy Centre, ESA, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
¹⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹¹ Johns Hopkins Universtity, Baltimore, MD 21218, USA
¹² Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
¹³ Ural Federal University, Astronomical Observatory, 620000 Ekaterinburg, Russian Federation

Received: 1 September 2015
Accepted: 23 May 2016

Abstract

Context. A discrepancy exists between the abundance of ammonia (NH₃) derived previously for the circumstellar envelope (CSE) of IRC+10216 from far-IR submillimeter rotational lines and that inferred from radio inversion or mid-infrared (MIR) absorption transitions.

Aims. To address the discrepancy described above, new high-resolution far-infrared (FIR) observations of both ortho- and para-NH₃ transitions toward IRC+10216 were obtained with Herschel, with the goal of determining the ammonia abundance and constraining the distribution of NH₃ in the envelope of IRC+10216.

Methods. We used the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel to observe all rotational transitions up to the J = 3 level (three ortho- and six para-NH₃ lines). We conducted non-LTE multilevel radiative transfer modelling, including the effects of near-infrared (NIR) radiative pumping through vibrational transitions. The computed emission line profiles are compared with the new HIFI data, the radio inversion transitions, and the MIR absorption lines in the ν₂ band taken from the literature.

Results. We found that NIR pumping is of key importance for understanding the excitation of rotational levels of NH₃. The derived NH₃ abundances relative to molecular hydrogen were (2.8 ± 0.5) × 10^-8 for ortho-NH₃ and (3.2   _-0.6^+0.7) × 10 ^-8 for para-NH₃, consistent with an ortho/para ratio of 1. These values are in a rough agreement with abundances derived from the inversion transitions, as well as with the total abundance of NH₃ inferred from the MIR absorption lines. To explain the observed rotational transitions, ammonia must be formed near to the central star at a radius close to the end of the wind acceleration region, but no larger than about 20 stellar radii (1σ confidence level).