1995A&A...293..833D

When modelling circumstellar envelopes by the use of radiative transfer codes a crucial input parameter is the absorption efficiency spectrum. However, the astronomical material which would permit a direct measurement of the optical properties of circumstellar grains is not generally available, and thus a realistic deduction of the complex dielectric function for the astronomical silicates requires the use of strong observational constraints. Between ∼8 and 23µm these are provided by the IRAS LRS catalogue. Out to 100µm the IRAS PSC furnishes an indication of the absorption profile of circumstellar envelopes. The intensity spectra alone, however, cannot directly furnish an unequivocal curve for the absorption efficiency since radiative transfer affects the band intensities and shapes to varying degrees. This implies that in attempts aimed at the determination of a reliable complex dielectric function radiative transfer is a key tool. The case of the optically thin circumstellar envelopes of supergiants is particularly well suited to this enterprise because all other parameters required in a self-consistant radiative transfer treatment of their envelopes are known within a reasonable error. Based on the radiative transfer modelling studies relative to a large sample of circumstellar envelopes (∼300 sources) undertaken by David & Papoular (1990A&A...237..425D) the complex dielectric function for circumstellar silicate grains is derived in the wavelength region from 0.35 to ∼135µm. The results of this computation are compared to the previously proposed dielectric function of Draine & Lee (1984ApJ...285...89D) derived under the assumed validity of the interstellar grain model of Mathis et al. (1977ApJ...217..425M). The discrepancies between these two complex dielectric functions are argued to be due to the different procedures used in their derivation and to the different observational constraints imposed. We conclude that our new determination of the complex dielectric function for circumstellar silicates is a more faithful representation for the average optical properties of galactic circumstellar dust than those offered previously.