1 N. Copernicus Astronomical Center,
Rabiańska 8, 87-100 Toruń, Poland
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
Accepted: 23 May 2016
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).
Key words: stars: AGB and post-AGB / circumstellar matter / stars: carbon / stars: individual: IRC+10216
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. HIFI is the Herschel Heterodyne Instrument for the Far Infrared.
The reduced spectra (FITS files) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/592/A131
© ESO, 2016