Ammonia in circumstellar environment of V Cyg
  The HIFI instrument on board of the Herschel Space Observatory (HSO) has been
very successful in detecting molecular lines from circumstellar envelopes
around evolved stars, like massive red supergiants, Asymptotic Giant Branch
(AGB) and post-AGB stars, as well as planetary nebulae. Among others, ammonia
has been found in circumstellar envelopes of C-rich AGB stars in amounts that
significantly exceeded theoretical predictions for C-rich stars. Few scenarios
have been proposed to resolve this problem: formation of ammonia behind the
shock front, photochemical processes in the inner part of the envelope partly
transparent to UV background radiation due to the clumpy structure of the gas,
and formation of ammonia on dust grains. Careful analysis of observations may
help to put constraints on one or another mechanism of ammonia formation. Here,
we present results of the non-LTE radiative transfer modeling of ammonia
transitions including a crucial process of radiative pumping via v$_2$ = 1
vibrational band (at $\sim$10 $\mu$m) for V Cyg. Only ground-based ammonia
transition NH$_{3}$ J = 1$_{0}$ - 0$_{0}$ at 572.5 GHz has been observed by
HIFI. Therefore, to determine abundance of ammonia we estimate a
photodissociation radius of NH$_{3}$ using chemical model of the envelope
consistent with dust grain properties concluded from the spectral energy
distribution.
