A chemical inventory of the S-type AGB star chi Cygni based on Herschel/HIFI observations of circumstellar line emission The importance of non-LTE chemical processes in a dynamical region

Journal article, 2011

Context. S-type AGB stars (C/O approximate to 1) are thought to be transition objects from M-type (O-rich) AGB stars to carbon stars and as such are interesting objects in themselves. Of particular interest is to determine accurate circumstellar properties and molecular abundances, due to their predicted sensitivity to the photospheric C/O-ratio. Aims. Presented here are new sensitive sub-millimetre line observations of molecules towards the S-type AGB star chi Cyg, using the HIFI instrument on-board the Herschel Space Observatory. The observed lines predominantly probe warm gas relatively close to the central star. Methods. Detailed, non-LTE, radiative transfer modelling has been used in order to interpret the circumstellar molecular line observations performed using HIFI, assuming a spherically symmetric, smooth, accelerating wind. Results. Lines from common molecules such as H2O, CO and SiO, which are expected to be abundant in an S-type AGB star, are clearly detected (as well as some of their isotopologues) in the HIFI spectra. In addition, we detect lines from carbon-bearing molecules such as HCN and CN. The CO line modelling indicates that the mass-loss rate has not undergone any significant modulations during the past approximate to 1000 yr. The derived o-H2O fractional abundance is approximate to 7x 10(-6), i.e., lower than those obtained for a small sample of M-type AGB stars but higher than what has been derived for a few carbon stars. We further obtain a p-H2O fractional abundance of approximate to 5 x 10(-6) giving an o/p-ratio of approximate to 1.4. Molecular line cooling is dominated by H2O only in a region close to the star (approximate to 6x 10(14) cm). The SiO abundance is estimated to be approximate to 1 x 10(-5). The (CO)-C-12/(CO)-C-13 ratio is 43 +/- 6. The high-excitation rotational lines clearly probe the acceleration region of the stellar wind (less than or similar to 2 x 10(15) cm) and put constraints on dynamical wind models. We are unable to fit consistently the combined ground-based and HIFI data for HCN and CN. Conclusions. The derived H2O abundance is reasonably consistent with recent chemical model predictions and so is the SiO abundance. The o/p-ratio of approximate to 1.4 supports a chemical formation under non-LTE conditions for the H2O molecules, and the presence of carbon-bearing molecules at relatively large abundances is also indicative of the importance of non-LTE chemical processes in regulating the circumstellar chemistry. The velocity field derived from the molecular line modelling is consistent with that obtained from solving for the wind dynamics through the coupled momentum equations of the dust and gas particles.