Optical constants of refractory oxides at high temperatures

Simon Zeidler; Thomas Posch; Harald Mutschke

doi:10.1051/0004-6361/201220459

paper

Optical constants of refractory oxides at high temperatures

arXiv:1304.1717 · doi:10.1051/0004-6361/201220459

Abstract

Many cosmic dust species, among them refractory oxides, form at temperatures higher than 300 K. Nevertheless, most astrophysical studies are based on the room-temperature optical constants of solids, such as corundum and spinel. A more realistic approach is needed for these materials, especially in the context of modeling late-type stars. We aimed at deriving sets of optical constants of selected, astrophysically relevant oxide dust species with high melting points. A high-temperature-high-pressure-cell and a Fourier-transform spectrometer were used to measure reflectance spectra of polished samples. For corundum (alpha-Al $_{2}$ O $_{3}$ ), spinel (MgAl $_{2}$ O $_{4}$ ), and alpha-quartz (SiO $_{2}$ ), temperature-dependent optical constants were measured from 300 K up to more than 900 K. Small particle spectra were also calculated from these data. All three examined oxides show a significant temperature dependence of their mid-IR bands. For the case of corundum, we find that the 13 $μ$ m emission feature - seen in the IR spectra of many AGB stars - can very well be assigned to this mineral species. The best fit of the feature is achieved with oblate corundum grains at mean temperatures around 550 K. Spinel remains a viable carrier of the 13 $μ$ m feature as well, but only for T < 300 K and nearly spherical grain shapes. Under such circumstances, spinel grains may also account for the 31.8 $μ$ m band that is frequently seen in sources of the 13 $μ$ m feature and which has not yet been identified with certainty.

arXiv commentAstronomy & Astrophysics, accepted, 26 February 2013. Article with 18 pages and 15 figures

Optical constants of refractory oxides at high temperatures

Abstract

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