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

Abstract

(abridged) 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. We have analysed molecular transitions of CO, SiO, and HCN measured with the APEX telescope and all three instruments on the Herschel Space Observatory, together with literature data. 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 have performed an analysis of the SiO and HCN molecular transitions in order to calculate their abundances. 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 outflow probed by our molecular data. For SiO, we find that the initial abundance lies between $5.5\times 10^{-5}$ and $6.0\times 10^{-5}$ w.r.t. H${}_2$. The abundance profile is constant up to $60\pm 10R_{\ast }$, after which it declines following a Gaussian profile with an $e$-folding radius of $3.5\pm 0.5\times 10^{13}$ cm. For HCN, we find an initial abundance of $5.0\times 10^{-7}$ w.r.t. H${}_2$. The Gaussian profile that describes the decline starts at the stellar surface and has an $e$-folding radius $r_e$ of $1.85\pm 0.05\times 10^{15}$ cm. We cannot to unambiguously identify the mechanism by which SiO is destroyed at $60\pm 10R_{\ast }$. The initial abundances found are larger 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.

16 pages, 13 figures, 7 tables, accepted for publication in Astronomy & Astrophysics

References in corpus (11)

• Radiative transfer in very optically thick circumstellar disks
• Too little radiation pressure on dust in the winds of oxygen-rich AGB stars
• A close halo of large transparent grains around extreme red giant stars
• Oxygen Chemistry in the Circumstellar Envelope of the Carbon-Rich Star IRC+10216
• Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles I. Theoretical model -- Mass-loss history unravelled in VY CMa
• Winds of M- and S-type AGB stars: an unorthodox suggestion for the driving mechanism
• Discovery of Time Variation of the Intensity of Molecular Lines in IRC+10216 in The Submillimeter and Far Infrared Domains
• The wind of W Hya as seen by Herschel - I. The CO envelope
• The variable mass loss of the AGB star WX Psc as traced by the CO J=1-0 through 7-6 lines and the dust emission
• H2O vapor excitation in dusty AGB envelopes. A PACS view of OH 127.8+0.0
• Dusty wind of W Hya. Multi-wavelength modelling of the present-day and recent mass-loss

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• An ALMA view of CS and SiS around oxygen-rich AGB stars
• Observation of narrow polar jets in the nascent wind of oxygen-rich AGB star EP Aqr
• Warm CO in evolved stars from the THROES catalogue. II. Herschel/PACS spectroscopy of C-rich envelopes