Steps toward interstellar silicate mineralogy III. The role of aluminium 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 in the 10 μm silicate band profiles, but also in the longer infrared wavelength range covered by ISO. This supports earlier conclusions that the concept of a universal cosmic silicate is inadequate and that it is reasonable to consider a wider variety of possible silicate analogues as the basis for an improved modelling of the observed spectra. In the course of an investigation of the role of aluminium in cosmic dust, aluminosilicate glasses (ASGs) have been suggested as interesting laboratory analogues for this purpose. In these glasses, silicon ions within the SiO4 tetrahedra are partly substituted by fourfold coordinated aluminium. In a new laboratory approach to the silicate dust problem, 13 ASG samples were prepared. Apart from magnesium and iron, the cosmically most abundant metals, sodium and calcium, were incorporated as cations. In this paper, these new silicate dust analogues are analytically and spectroscopically characterized. The spectroscopic results were obtained in the range from the UV to the far-infrared, in the millimetre wave range, and by Raman spectroscopy. Optical constants have been derived for the wavenumbers 1500–20 cm−1 (6.7–500 μm) from infrared reflectance measurements and for the frequencies 110– 75 GHz (2.7–4 mm) from angle-resolved millimetre-wave scattering at spherical samples from two of the ASGs. For the electronic and vibrational absorption features, the dependence on the aluminium/silicon substitution ratio and on the glass structure is discussed. Relations between the calculated (Rayleigh case) band positions, widths and strengths of the IR absorption bands and the chemical compositions are derived. The application of the new data to the reproduction of observed stardust spectra of the IRAS-LRS catalogue shows promising results.