A HIFI preview of warm molecular gas around chi Cyg : first detection of
  H2O emission toward an S-type AGB star
  A set of new, sensitive, and spectrally resolved, sub-millimeter line
observations are used to probe the warm circumstellar gas around the S-type AGB
star chi Cyg. The observed lines involve high rotational quantum numbers,
which, combined with previously obtained lower-frequency data, make it possible
to study in detail the chemical and physical properties of, essentially, the
entire circumstellar envelope of chi Cyg. The data were obtained using the HIFI
instrument aboard Herschel, whose high spectral resolution provides valuable
information about the line profiles. Detailed, non-LTE, radiative transfer
modelling, including dust radiative transfer coupled with a dynamical model,
has been performed to derive the temperature, density, and velocity structure
of the circumstellar envelope. We report the first detection of circumstellar
H2O rotational emission lines in an S-star. Using the high-J CO lines to derive
the parameters for the circumstellar envelope, we modelled both the ortho- and
para-H2O lines. Our modelling results are consistent with the velocity
structure expected for a dust-driven wind. The derived total H2O abundance
(relative to H2) is (1.1 +/- 0.2)E-5, much lower than that in O-rich stars. The
derived ortho-to-para ratio of 2.1 +/- 0.6 is close to the high-temperature
equilibrium limit, consistent with H2O being formed in the photosphere.
