  The main aim is to derive reliable mass-loss rates and circumstellar SiO
abundances for a sample of 40 S-type AGB stars based on new multi-transitional
CO and SiO radio line observations. In addition, the results are compared to
previous results for M-type AGB stars and carbon stars to look for trends with
chemical type. The circumstellar envelopes are assumed to be spherically
symmetric and formed by a constant mass-loss rate. The mass-loss rates are
estimated from fitting the CO observations using a non-local, non-LTE radiative
transfer code. Once the physical properties of the circumstellar envelopes are
determined, the same radiative transfer code is used to model the observed SiO
lines in order to derive circumstellar abundances and the sizes of the SiO
line-emitting regions. We have estimated mass-loss rates of 40 S-type AGB stars
and find that the derived mass-loss rates have a distribution that resembles
those previously derived for similar samples of M-type AGB stars and carbon
stars. The estimated mass-loss rates also correlate well with the corresponding
expansion velocity. In all, this indicates that the mass loss is driven by the
same mechanism in all three chemical types of AGB stars. In addition, we have
estimated the circumstellar fractional abundance of SiO relative to H2 in 26 of
the sample S-type AGB stars. The derived SiO abundances are, on average, about
an order of magnitude higher than predicted by stellar atmosphere thermal
equilibrium chemistry, indicating that non-equilibrium chemical processes
determines the abundance of SiO in the circumstellar envelope. Moreover, a
comparison with the results for M-type AGB stars and carbon stars show that for
a certain mass-loss rate, the circumstellar SiO abundance seems independent
(although with a large scatter) of the C/O-ratio.
