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New SiS multi-transition (sub-)millimetre line observations of a sample ofAGB stars with varying photospheric C/O-ratios and mass-loss rates arepresented. A combination of low- and high-energy lines are important inconstraining the circumstellar distribution of SiS molecules. A detailedradiative transfer modelling of the observed SiS line emission is performed,including the effect of thermal dust grains in the excitation analysis. We findthat the circumstellar fractional abundance of SiS in these environments has astrong dependence on the photospheric C/O-ratio as expected from chemicalmodels. The carbon stars (C/O>1) have a mean fractional abundance of 3.1E-6,about an order of magnitude higher than found for the M-type AGB stars (C/O<1)where the mean value is 2.7E-7. These numbers are in reasonable agreement withphotospheric LTE chemical models. SiS appears to behave similar to SiO in termsof photodissociation in the outer part of the circumstellar envelope. Incontrast to previous results for the related molecule SiO, there is no strongcorrelation of the fractional abundance with density in the CSE, as would bethe case if freeze-out onto dust grains were important. However, possibletime-variability of the line emission in the lower J transitions and thesensitivity of the line emission to abundance gradients in the inner part ofthe CSE may mask a correlation with the density of the wind. There areindications that the SiS fractional abundance could be significantly highercloser to the star which, at least in the case of M-type AGB stars, wouldrequire non-equilibrium chemical processes.

The abundance of SiS in circumstellar envelopes around AGB stars