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

Abstract

<i>Aims. <i/>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 <i>χ<i/> 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 <i>χ<i/> Cyg.<i>Methods. <i/>The data were obtained using the HIFI instrument aboard <i>Herschel<i/>, 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.<i>Results. <i/>We report the first detection of circumstellar H<sub>2<sub/>O rotational emission lines in an S-star. Using the high-<i>J<i/> CO lines to derive the parameters for the circumstellar envelope, we modelled both the ortho- and para-H<sub>2<sub/>O lines. Our modelling results are consistent with the velocity structure expected for a dust-driven wind. The derived total H<sub>2<sub/>O abundance (relative to H<sub>2<sub/>) is (1.1<i>±<i/>0.2) × 10<sup>-5<sup/>, much lower than that in O-rich stars. The derived ortho-to-para ratio of 2.1<i>±<i/>0.6 is close to the high-temperature equilibrium limit, consistent with H<sub>2<sub/>O being formed in the photosphere.