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The main aim is to derive reliable mass-loss rates and circumstellar SiOabundances for a sample of 40 S-type AGB stars based on new multi-transitionalCO and SiO radio line observations. In addition, the results are compared toprevious results for M-type AGB stars and carbon stars to look for trends withchemical type. The circumstellar envelopes are assumed to be sphericallysymmetric and formed by a constant mass-loss rate. The mass-loss rates areestimated from fitting the CO observations using a non-local, non-LTE radiativetransfer code. Once the physical properties of the circumstellar envelopes aredetermined, the same radiative transfer code is used to model the observed SiOlines in order to derive circumstellar abundances and the sizes of the SiOline-emitting regions. We have estimated mass-loss rates of 40 S-type AGB starsand find that the derived mass-loss rates have a distribution that resemblesthose previously derived for similar samples of M-type AGB stars and carbonstars. The estimated mass-loss rates also correlate well with the correspondingexpansion velocity. In all, this indicates that the mass loss is driven by thesame mechanism in all three chemical types of AGB stars. In addition, we haveestimated the circumstellar fractional abundance of SiO relative to H2 in 26 ofthe sample S-type AGB stars. The derived SiO abundances are, on average, aboutan order of magnitude higher than predicted by stellar atmosphere thermalequilibrium chemistry, indicating that non-equilibrium chemical processesdetermines the abundance of SiO in the circumstellar envelope. Moreover, acomparison with the results for M-type AGB stars and carbon stars show that fora certain mass-loss rate, the circumstellar SiO abundance seems independent(although with a large scatter) of the C/O-ratio.

Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances