  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.
  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)x10^{-8} for ortho-NH3 and
(3.2^{+0.7}_{-0.6})x10^{-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 (1
sigma confidence level).
