2009A&A...501..251T

To trace the source of the unique 13, 19.5, and 28µm emission features in the spectra of oxygen-rich circumstellar shells around AGB stars, we have compared dust extinction spectra obtained by aerosol measurements. We have measured the extinction spectra for 19 oxide powder samples of eight different types, such as Ti-compounds (TiO, TiO₂, Ti₂O₃, Ti₃O₅, Al₂TiO₅, CaTiO₃), α-, γ-, χ-δ-κ-Al₂O₃, and MgAl₂O₄ in the infrared region (10-50µm) paying special attention to the morphological (size, shape, and agglomeration) effects and the differences in crystal structure. Anatase (TiO₂) particles with rounded edges are the possible 13, 19.5 and 28 µm band carriers as the main contributor in the spectra of AGB stars, and spherically shaped nano-sized spinel and Al₂TiO₅ dust grains are possibly associated with the anatase, enhancing the prominence of the 13 µm feature and providing additional features at 28µm. The extinction data sets obtained by the aerosol and CsI pellet measurements have been made available for public use at url http://elbe.astro.uni-jena.de .