Steps toward interstellar silicate mineralogy

Abstract

Amorphous silicate particles are generally assumed to be the maindust component in the envelopes of oxygen-rich evolved stars andmay be considered the precursors of the pure crystalline enstatiteand forsterite particles detected by ISO. We present optical constantsin the broad wavelength range 0.2500 μm for a unique series ofpure amorphous Mg-silicates (Mg/Si in the range 0.72.4). They havebeen prepared by the sol-gel process, a chemical technique basedon the condensation of Mg- and Si-hydroxides in a liquid phase. Thesalient feature of these Mg-silicates is the very small content ofSiOH bonds in the silicate network, which considerably reduces theactivation energy of crystallization and, thus, decreases the temperaturethreshold for crystallization as well as crystallization time. Theastrophysical relevance of our sol-gel silicates is shown by a comparisonof optically thin model spectra based on dust emissivities with ISO-SWSspectra of AGB stars and with 10 μm emission profiles of suchstars obtained by ground-based spectroscopy. As paradigmatic casesof AGB spectra with respect to the appearance of the silicate bands,TY Dra (slender bands and deep trough between them) and R Cas (broadbands and widely filled-up trough) were used, for which ISO-SWS spectraare available. The dust emissivity derived from TY Dra can be excel-lently reproduced by the models, suggesting that the dust grainsconsist indeed of pure amorphous Mg-silicates. Satisfactory agreementwas also found with the mean 10 μm profiles of some groups ofAGB stars and supergiants. Spectra with strong dust emission in thesilicate trough like R Cas require additional contributions by otherdust components, probably oxides. A rough orientation on the spectralproperties of such potential trough opacity contributors has beenobtained by subtracting a pure silicate spectrum (TY Dra) from aspectrum with a nearly filled trough and a less pronounced 20 μmband (R Cas). In agreement with other amorphous silicates, the spectralindex of the new silicate analogues amounts to 2.