Sulphur molecules in the circumstellar envelopes of M-type AGB stars

Abstract

The sulphur compounds SO and SO${}_2$ have not been widely studied in the circumstellar envelopes of asymptotic giant branch (AGB) stars. By presenting and modelling a large number of SO and SO${}_2$ lines in the low mass-loss rate M-type AGB star R Dor, and modelling the available lines of those molecules in a further four M-type AGB stars, we aim to determine their circumstellar abundances and distributions. We use a detailed radiative transfer analysis based on the accelerated lambda iteration method to model circumstellar SO and SO${}_2$ line emission and molecular data files for both SO and SO${}_2$ that are more extensive than those previously available. Using 17 SO lines and 98 SO2 lines to constrain our models for R Dor, we find an SO abundance of 6.7x10${}^{-6}$ and an SO${}_2$ abundance of 5x10${}^{-6}$ with both species having high abundances close to the star. We also modelled ${}^{34}$SO and found an abundance of 3.1x10${}^{-7}$, giving an ${}^{32}$SO/${}^{34}$SO ratio of 21.6. We derive similar results for the circumstellar SO and SO${}_2$ abundances and their distributions for the low mass-loss rate object W Hya. For these stars, the circumstellar SO and SO${}_2$ abundances are much higher than predicted by chemical models and these two species may account for all available sulphur. For the higher mass-loss rate stars, we find shell-like SO distributions with peak abundances that decrease and peak abundance radii that increase with increasing mass-loss rate. The positions of the peak SO abundance agree very well with the photodissociation radii of H${}_2$O. We find evidence that SO is most likely through the photodissociation of H${}_2$O and the subsequent reaction between S and OH. The S-bearing parent molecule appears not to be H${}_2$S. The SO${}_2$ models suggest an origin close to the star for this species, also disagreeing with current chemical models.

25 pages

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