Infrared Properties of Solid Titanium Oxides: Exploring Potential Primary Dust Condensates

We present optical constants and opacities of solid TiO2 ,T i 2O3, magnesium and calcium titanates, largely based on laboratory measurements. These dust species deserve interest as potential primary condensates in oxygen-rich dusty environments. Of the three known solid TiO2 phases, only one (rutile) has been extensively studied so far with respect to its mid-IR optical properties. We compare these with our measurements of the optical constants of anatase, brookite, and CaTiO3. Furthermore, for several Mg-Ti-oxides, powder transmission spectra are shown. While all known TiO2 modifications have their strongest bands between 13 and 13.5 lm (for spherical particles), CaTiO3, MgTiO3, and other Mg titanates have principal maxima of their absorption coefficients between 14 and 19 lm. This makes a spectroscopic identification of circumstellar Ti oxide particles rather difficult, because both the 13 and the 14–19 lm region are crowded with other features in the spectra of oxygen-rich circumstellar shells. Subject headings: circumstellar matter — methods: laboratory — molecular data — solar system: formation — stars: AGB and post-AGB — stars: mass loss

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