Dust envelopes of two S-type AGB stars

The ISO-SWS01 spectra of two S stars, IRAS 15492+4837 and IRAS19486+3247, have been investigated with the radiative transfer code DUSTY. For the two stars, the mass-loss rates determined from DUSTY are smaller than the values derived from CO emission, which might indicate that the mass-loss rates of the two stars was higher in the past. The ∼10 μm dust feature is well fitted with a combination of amorphous alumina and melilite. Corundum and Mg0.1Fe0.9O reproduce the 13 and 19.8 μm emission features of IRAS 15492+4837, respectively. We also try to use spinel to fit the 13 and 32 μm features of IRAS 15492+4837, but the two features cannot be well fitted at the same time though spinel reproduces the 13 μm feature, and furthermore it also reproduces a 16.8 μm feature not observed on the spectra of IRAS 15492+4837, which indicates that the probability of spinel as the responsible for the 13 μm feature is less than corundum.