A HIFI preview of warm molecular gas around χ Cygni: first detection of H2O emission toward an S-type AGB star

摘要

Aims. A set of new, sensitive, and spectrally resolved,sub-millimeter line observations are used to probe the warmcircumstellar gas around the S-type AGB star Cyg. The observed lines involve high rotational quantum numbers, which,combined with previously obtained lower-frequency data, make itpossible to study in detail the chemical and physical properties of,essentially, the entire circumstellar envelope of Cyg. Methods. The data were obtained using the HIFI instrument aboard Herschel,whose high spectral resolution provides valuable information about theline profiles. Detailed, non-LTE, radiative transfer modelling,including dust radiative transfer coupled with a dynamical model, hasbeen performed to derive the temperature, density, and velocitystructure 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 ,much lower than that in O-rich stars. The derived ortho-to-para ratio of is close to the high-temperature equilibrium limit, consistent with H2O being formed in the photosphere. Key words: stars: AGB and post-AGB - circumstellar matter - stars: kinematicsand dynamics - stars: individual: Cyg - stars: late-type - stars: mass-loss