In order to determine the composition of the dust in the circumstellar envelopes of oxygen-rich asymptotic giant branch ( AGB ) stars we have computed a grid of modust radiative-transfer models for a range of dust compositions , mass-loss rates , dust shell inner radii and stellar parameters .
We compare the resulting colours with the observed oxygen-rich AGB stars from the SAGE-Spec Large Magellanic Cloud ( LMC ) sample , finding good overall agreement for stars with a mid-infrared excess .
We use these models to fit a sample of 37 O-rich AGB stars in the LMC with optically thin circumstellar envelopes , for which 5–35- \mu m Spitzer infrared spectrograph ( IRS ) spectra and broadband photometry from the optical to the mid-infrared are available .
From the modelling , we find mass-loss rates in the range \sim 8 \times 10 ^ { -8 } to 5 \times 10 ^ { -6 } M _ { \odot } \mathrm { yr } ^ { -1 } , and we show that a grain mixture consisting primarily of amorphous silicates , with contributions from amorphous alumina and metallic iron provides a good fit to the observed spectra .
Furthermore , we show from dust models that the AKARI [ 11 ] – [ 15 ] versus [ 3.2 ] – [ 7 ] colour-colour diagram , is able to determine the fractional abundance of alumina in O-rich AGB stars .