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

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.

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.

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.

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 [Formula: see text] 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).