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

摘要

Aims. 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. Methods. We use a detailed radiative transfer analysis based on the accelerated lambda iteration method to model circumstellar SO and SO_(2) line emission. We use molecular data files for both SO and SO_(2) that are more extensive than those previously available. Results. Using 17 SO lines and 98 SO_(2) lines to constrain our models for R Dor, we find an SO abundance of (6.7 ± 0.9) × 10~(-6) and an SO_(2) abundance of 5 × 10~(-6) with both species having high abundances close to the star. We also modelled ~(34) SO and found an abundance of (3.1 ± 0.8) × 10~(-7) , giving an ~(32) SO/ ~(34) SO ratio of 21.6 ± 8.5 . 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 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 also modelled SO_(2) in two higher mass-loss rate stars but our models for these were less conclusive. Conclusions. We conclude that for the low mass-loss rate stars, the circumstellar SO and SO_(2) abundances are much higher than predicted by chemical models of the extended stellar atmosphere. These two species may also account for all the available sulphur. For the higher mass-loss rate stars we find evidence that SO is most efficiently formed in the circumstellar envelope, most likely through the photodissociation of H_(2) O and the subsequent reaction between S and OH. The S-bearing parent molecule does not appear to be H_(2) S. The SO_(2) models for the higher mass-loss rate stars are less conclusive, but suggest an origin close to the star for this species. This is not consistent with current chemical models. The combined circumstellar SO and SO_(2) abundances are significantly lower than that of sulphur for these higher mass-loss rate objects.