Abstract

Abstract : A summary of observations of the angular diameters of the shells around evolved red giants is reported. Many of these sources are partially resolved with a ground-based 2.4-m telescope. We have concentrated our efforts in the 8 - 13 micron spectral range because this corresponds to the emission excesses observed in the spectra of oxygen-rich stars. These excesses are commonly thought to arise from small 'silicate' mineral grains in the circumstellar shells. The shell structure has been fully resolved for the bright supergiant alpha Orionis. A study of the results shows that the opacity of the silicate mineral grains around that star is similar to that inferred for other evolved stars and for the Trapezium.

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