The physics and chemistry of circumstellar envelopes of S stars on the AGB

S-type stars on the AGB are thought to be in a brief transitional phase as they evolve from oxygen-rich M-type stars into carbon stars, through the dredge-up of carbon from He shell burning. As possible transition objects, S stars might help achieve a deeper understanding of the chemical evolution as a star ascends the AGB, as well as shed light on the mass-loss mechanism. We have initiated a large survey of 40 S stars in circumstellar radio line emission from common molecules such as CO, CS, SiO, SiS, and HCN. Detailed radiative transfer modelling of multi-transition CO radio line data towards the sample stars shows that the mass-loss rate distribution of S stars is similar to those found for M-type AGB stars and carbon stars. Initial results from modelling of circumstellar SiO line emission are also presented.