Amorphous magnesium silicate

About 85-90 % of the dust condensing in the envelopes of oxygen-rich evolved stars consist of amorphous magnesium or magnesium-iron silicates [6]. Optical constants ($n,k$) of stoichiometric and nonstoichimetric magnesium silicates with Mg/Si ratios from 0.7 to 2.4 produced by the sol-gel method have been derived from reflection measurements by a combination of Kramers-Kronig analysis (KKR) and Lorentz-oscillator fit method (see Fig. ). The calculated absorption coefficients show that MgO influences the position of the 10 and 20$\mu$m band. With increasing MgO content the 10 $\mu$m band is shifted to longer wavelengths whereas the 20 $\mu$m band is shifted in the opposite direction.

Figure: Left panel: Imaginary part of the refractive index for amorphous Mg$_{0.7}$SiO$_{2.7}$ (dotted line) and Mg$_{2.4}$SiO$_{4.4}$ (solid line). Right panel: Absorption efficiency normalized by particle radius calculated for a continuous distribution of ellipsoidal grain shapes (CDE) composed of the same materials.

The comparison between differently produced magnesium silicates demonstrates the existence of varying amorphous magnesium silicates material with differences in the internal structures. Based upon these results one has to conclude that the amorphous state of any magnesium silicate is not unique but there exist different possibilities for structural arrangements of subunits in the amorphous silicate structure, similar to the varying structures of amorphous carbon [10]. The astrophysical usefulness of these sol-gel silicates was tested by comparison of optically thin model spectra based on the new optical data with the dust emissivity derived from ISO-SWS spectra of AGB stars in the range between 8-30 $\mu$m. The dust emissivity derived from TY Dra, an evolved dust forming star, can excellently be reproduced by the models, sugessting that the dust grains consist indeed of pure amorphous Mg silicate.