Steps toward interstellar silicate mineralogy: VII. Spectral properties and crystallization behaviour of magnesium silicates produced by the sol-gel method

Abstract

Amorphous silicate particles are generally assumed to be the main dust component in the envelopes of oxygen-rich evolved stars and may be considered the precursors of the pure crystalline enstatite and forsterite particles detected by ISO. We present optical constants in the broad wavelength range 0.2–500 μm for a unique series of pure amorphous Mg-silicates (Mg/Si in the range 0.7–2.4). They have been prepared by the sol-gel process, a chemical technique based on the condensation of Mg- and Si-hydroxides in a liquid phase. The salient feature of these Mg-silicates is the very small content of Si–OH bonds in the silicate network, which considerably reduces the activation energy of crystallization and, thus, decreases the temperature threshold for crystallization as well as crystallization time. The astrophysical relevance of our sol-gel silicates is shown by a comparison of optically thin model spectra based on dust emissivities with ISO-SWS spectra of AGB stars and with 10 μm emission profiles of such stars obtained by ground-based spectroscopy. As paradigmatic cases of AGB spectra with respect to the appearance of the silicate bands, TY Dra (slender bands and deep trough between them) and R Cas (broad bands and widely filled-up trough) were used, for which ISO-SWS spectra are available. The dust emissivity derived from TY Dra can be excellently reproduced by the models, suggesting that the dust grains consist indeed of pure amorphous Mg-silicates. Satisfactory agreement was also found with the mean 10 μm profiles of some groups of AGB stars and supergiants. Spectra with strong dust emission in the silicate trough like R Cas require additional contributions by other dust components, probably oxides. A rough orientation on the spectral properties of such potential trough opacity contributors has been obtained by subtracting a pure silicate spectrum (TY Dra) from a spectrum with a nearly filled trough and a less pronounced 20 μm band (R Cas). In agreement with other amorphous silicates, the spectral index of the new silicate analogues amounts to -2.