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We investigate the composition of the solid-state materials in the windsaround S-type AGB stars. The S stars produce dust in their wind that bears aresemblance to the dust produced in some O-rich AGB stars. However, thereported resemblance is mostly based on IRAS/LRS spectra with limited spectralresolution, sensitivity, and wavelength coverage. We investigate the dustcomposition around S stars using ISO/SWS data that surpass the previous studiesin terms of spectral resolution and wavelength coverage. We compare the dustspectra from the 9 sources with the O-rich AGB spectra and a subset of Msuper-giants. We constructed average dust emission spectra of the differentcategories. We report the discovery of several previously unreported dustemission features in the S star spectra. The long wavelength spectra of W Aqland pi1 Gru exhibit the "30" micrometer feature attributed to MgS. Two sourcesexhibit a series of emission bands between 20 and 40 micrometer that wetentatively ascribe to Diopside. We show that the 10-20 micrometer spectra ofthe S stars are significantly different from the O-rich AGB stars. The O-richstars exhibit a structured emission feature that is believed to arise fromamorphous silicate and aluminium-oxide. The S stars lack the substructure foundin the O-rich stars. Instead they show a smooth peak with a varyingpeak-position from source to source. We suggest that this feature is caused bya family of related material, whose exact composition determines the peakposition. The observed trend mimics the laboratory trend of non-stoichiometricsilicates. In this scenario the degree of non-stoichiometry is related to theMg to SiO4 ratio, in other words, to the amount of free O available during thedust grain growth.

An ISO/SWS study of the dust composition around S stars