Circumstellar Oxide Particles and their Infrared Features

Author(s): Thomas Posch, Ingrid Hodous, Wilhelm Nöbauer, Franz Kerschbaum, Harald Mutschke
Abstract: Oxide particles are -- together with silicates -- a major component of the dust shells of Asymptotic Giant Branch (AGB) stars, especially of those which have low mass loss rates. During the past years, it has been possible to identify these grain species both by infrared spectroscopy and by investigations of presolar meteoritic grains. The following materials are presented here as probable stardust components: aluminium oxide, magnesium-aluminium oxide, (calcium-)titanium and magnesium-iron oxides. Amorphous Al₂O₃ (alumina) produces a broad emission band peaking at 11--12 microns. Such a feature is indeed observed in the spectra of oxygen-rich AGB stars with low mass loss rates. We show examples of such ISO spectra and compare them with the emissivity of alumina, obtained by condensation experiments as well as by a sol-gel technique. An alternative carrier of the 11--12 micron band is SiC, which, however, is expected to form mainly in carbon-rich environments. In S-stars, both SiC and alumina are likely to be carriers of the 11--12 micron feature. MgAl₂O₄ (spinel) is very likely to be the carrier of the 13.0 micron feature which has originally been attributed to alpha-Al2O3. On the basis of ISO spectra, in addition to the 13 micron feature, a weak 16.8 micron and a rather prominent 31.8 micron feature were detected. This supports the idea that spinel is indeed the carrier of all three bands. New measurements on the dependence of the 31.8 micron feature profile on the temperature of the radiating particles are presented. All known TiO₂ modifications (rutile, anatase, brookite) have their strongest small-particle resonances between 13 and 13.5 microns, as our laboratory measurements have shown. Another mineral which is very likely to form around AGB stars is CaTiO3 (perovskite). Unfortunately, its optical constants are as yet unknown. We present optical data of this substance and discuss whether it can be spectroscopically identified in stardust spectra. Finally, we point out that (Mg,Fe)O (magnesiowustite) is almost certainly the carrier of a strong band centered at 19.5 microns. The influence of the stoichiometry and of the particle shape on the small particle spectra of magnesiowustites are presented.
Organisation(s): Department of Astrophysics
External organisation(s): Friedrich-Schiller-Universität Jena
Pages: 147
Publication date: 2003
Austrian Fields of Science 2012: 103003 Astronomy, 103004 Astrophysics
Portal url: https://ucrisportal.univie.ac.at/en/publications/8e7ec190-f33a-47ce-b7e5-2b7d9bdf57a0

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