Chemical content of the circumstellar envelope of the oxygen-rich AGB star R Doradus: Non-LTE abundance analysis of CO, SiO, and HCN

Journal article, 2018

Context. The stellar outflows of low- to intermediate-mass stars are characterised by a rich chemistry. Condensation of molecular gas species into dust grains is a key component in a chain of physical processes that leads to the onset of a stellar wind. In order to improve our understanding of the coupling between the micro-scale chemistry and macro-scale dynamics, we need to retrieve the abundance of molecules throughout the outflow. Aims. Our aim is to determine the radial abundance profile of SiO and HCN throughout the stellar outflow of R Dor, an oxygen-rich AGB star with a low mass-loss rate. SiO is thought to play an essential role in the dust-formation process of oxygen-rich AGB stars. The presence of HCN in an oxygen-rich environment is thought to be due to non-equilibrium chemistry in the inner wind. Methods. We analysed molecular transitions of CO, SiO, and HCN measured with the APEX telescope and all three instruments on the Herschel Space Observatory, together with data available in the literature. Photometric data and the infrared spectrum measured by ISO-SWS were used to constrain the dust component of the outflow. Using both continuum and line radiative transfer methods, a physical envelope model of both gas and dust was established. We performed an analysis of the SiO and HCN molecular transitions in order to calculate their abundances. Results. We have obtained an envelope model that describes the dust and the gas in the outflow, and determined the abundance of SiO and HCN throughout the region of the stellar outflow probed by our molecular data. For SiO, we find that the initial abundance lies between 5.5 × 10 -5 and 6.0 × 10 -5 with respect to H 2 . The abundance profile is constant up to 60 ± 10 R, after which it declines following a Gaussian profile with an e-folding radius of 3.5 ± 0.5 × 10 13 cm or 1.4 ± 0.2 R. For HCN, we find an initial abundance of 5.0 × 10 -7 with respect to H 2 . The Gaussian profile that describes the decline starts at the stellar surface and has an e-folding radius r e of 1.85 ± 0.05 × 10 15 cm or 74 ± 2 R. Conclusions. We cannot unambiguously identify the mechanism by which SiO is destroyed at 60 ± 10 R. The initial abundances found are higher than previously determined (except for one previous study on SiO), which might be due to the inclusion of higher-J transitions. The difference in abundance for SiO and HCN compared to high mass-loss rate Mira star IK Tau might be due to different pulsation characteristics of the central star and/or a difference in dust condensation physics.