STEPS TOWARD INTERSTELLAR SILICATE MINERALOGY - III - THE ROLE OF ALUMINUM IN CIRCUMSTELLAR AMORPHOUS SILICATES

It is a well-known fact that the spectra of oxygen-rich circumstellar dust envelopes around evolved stars show great diversity, especially i n the 10 mu m silicate band profiles, but also in the longer infrared wavelength range covered by ISO. This supports earlier conclusions tha t the concept of a universal cosmic silicate is inadequate and that it is reasonable to consider a wider variety of possible silicate analog ues as the basis for an improved modelling of the observed spectra. In the course of an investigation of the role of aluminium in cosmic dus t, aluminosilicate glasses (ASGs) have been suggested as interesting l aboratory analogues for this purpose. In these glasses, silicon ions w ithin the SiO4 tetrahedra are partly substituted by fourfold coordinat ed aluminium. In a new laboratory approach to the silicate dust proble m, 13 ASG samples were prepared. Apart from magnesium and iron, the co smically most abundant metals, sodium and calcium, were incorporated a s cations. In this paper, these new silicate dust analogues are analyt ically and spectroscopically characterized. The spectroscopic results were obtained in the range from the UV to the far-infrared, in the mil limetre wave range, and by Raman spectroscopy. Optical constants have been derived for the wavenumbers 1500-20 cm(-1) (6.7-500 mu m) from in frared reflectance measurements and for the frequencies 110-75 GHz (2. 7-4 mm) from angle-resolved millimetre-wave scattering at spherical sa mples from two of the ASGs. For the electronic and vibrational absorpt ion features, the dependence on the aluminium/silicon substitution rat io and on the glass structure is discussed. Relations between the calc ulated (Rayleigh case) band positions, widths and strengths of the IR absorption bands and the chemical compositions are derived. The applic ation of the new data to the reproduction of observed stardust spectra of the IRAS-LRS catalogue shows promising results.