PT Journal
AU Justtanont, K
   Decin, L
   Schöier, FL
   Maercker, M
   Olofsson, H
   Bujarrabal, V
   Marston, AP
   Teyssier, D
   Alcolea, J
   Cernicharo, J
   Dominik, C
   de Koter, A
   Melnick, G
   Menten, K
   Neufeld, D
   Planesas, P
   Schmidt, M
   Szczerba, R
   Waters, R
   de Graauw, T
   Whyborn, N
   Finn, T
   Helmich, F
   Siebertz, O
   Schmülling, F
   Ossenkopf, V
   Lai, R
TI A HIFI preview of warm molecular gas around χ Cygni: first detection of H2O emission toward an S-type AGB star
SO Astron. Astrophys.
PY 2010
BP L6
VL 521
DI 10.1051/0004-6361/201015092
DE HEB mixer applications; HIFI; Herschel; stars: AGB and post-AGB / circumstellar matter / stars: kinematics and dynamics / stars: individual: χ Cyg / stars: late-type / stars: mass-loss
AB Aims. 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 χ 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 χ Cyg.Methods. 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.Results. 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) × 10-5, 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.
ER