A HIFI preview of warm molecular gas around χ Cygni: first detection of H2O emission toward an S-type AGB star
de Graauw, Th.; Olofsson, H.; Marston, A. P.; Dominik, C.; Teyssier, D.; Ossenkopf, V.; Justtanont, K.; Planesas, P.; Alcolea, J.; de Koter, A.; Cernicharo, J.; Szczerba, R.; Melnick, G.; Schmidt, M.; Decin, L.; Schöier, F. L.; Bujarrabal, V.; Menten, K.; Siebertz, O.; Schmülling, F.; Whyborn, N.; Neufeld, D.; Helmich, F.; Maercker, M.; Waters, R.; Finn, T.; Lai, R.
Sweden, Belgium, Netherlands, Germany, Spain, United States, Chile, Poland, Ireland
Abstract
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.