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

Abstract

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.

Sulphur-bearing molecules in AGB stars

“…They find a much lower H 2 S abundance than our radiative transfer results for IK Tau (and indeed, all five of our sources), which are a few orders of magnitude higher. The Li et al (2016) results are also in disagreement with the observational SO results found by Danilovich et al (2016) for IK Tau, which show a lower inner abundance of SO with a peak at ∼ 10 16 cm. The Agúndez et al (2010) study covers a similar region of the CSE -beginning at 10 14 cm, further inwards than Li et al (2016) -but primarily focuses on the effect of clumpiness in the CSE.…”

“…ALI has been previously described in detail by Maercker et al (2008) and Schöier et al (2011) and has been implemented to model molecular emission from a wide variety of molecules in those studies and in Danilovich et al (2014Danilovich et al ( , 2016. Our models assume a smoothly accelerating and spherically symmetric CSE created by mass lost at a constant rate by the central AGB star.…”

“…Furthermore, sulphur is not formed in AGB stars nor their main sequence progenitors, so the total amount of sulphur-bearing molecules can be constrained based on upper limits from galactic and solar abundances for sulphur. Indeed, Danilovich et al (2016) began to do this based on a thorough study of SO and SO 2 in a small sample of M-type AGB stars.…”

First detection of methanol towards a post-AGB object, HD 101584

“…The SO (1 × 10 −6 ), SiO (5 × 10 −6 ), and CS (1 × 10 −6 ; after correcting the 13 CS abundance by the estimated 12 C/ 13 C ≈ 10) abundances all lie in the range reported for AGB CSEs (Danilovich et al 2016;Schöier et al 2013;Velilla Prieto et al 2017), while the H 2 CO abundance (2 × 10 −6 ; assuming an orthoto-para ratio of 3) is about an order of magnitude higher than in IK Tau (Velilla Prieto et al 2017). CH 3 OH has an estimated abundance of 3 × 10 −6 (assuming an E-to-A ratio of 1), almost two orders of magnitude higher than towards the supergiant IRC+10420 (Quintana-Lacaci et al 2016).…”

Circumstellar ammonia in oxygen-rich evolved stars

“…We note that the abundance of sulphur oxides is yet another mystery in circumstellar chemistry of O-rich stars (e.g. Yamamura et al 1999;Fu et al 2012;Adande et al 2013;Danilovich et al 2016;Velilla Prieto et al 2017). Our results may be indicative of some violent processes, such as shocks, in the formation of NH 3 .…”