摘要：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 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).
关键词：stars: AGB and post-AGB;circumstellar matter;stars: carbon;stars: individual: IRC+10216
