On the origin of the 13 mu m feature - A study of ISO-SWS spectra of oxygen-rich AGB stars

We have derived a mean profile of the 13 mu m emission feature from 11 ISO spectra of oxygen-rich AGE stars and present an overview of the mineral spe cies that could account for it. Our results can be summarized as follows: i ) Spherical particles of alpha-Al2O3 (corundum) have a sharp emissivity max imum peaking at 12.7 mu m The difference of 0.3 mu m in the peak position c ompared to the mean observed band profile is a serious obstacle to assignin g the 13 mu m dust feature to this dust species. ii) From a continuous dist ribution of ellipsoidal alpha-Al2O3-particles, a broad emissivity profile p eaking at 13-14 mu m emerges. It is difficult to reconcile the large width of this profile with the width of the observed 13 mu m band. iii) The most prominent emissivity maximum of TiO2 (rutile) is located at 13.5 mu m; its width is larger and its strength three times smaller than the emissivity ma ximum of corundum. iv) Core-mantle-grains composed of rutile and corundum o r of corundum and amorphous olivine can both produce, in a certain domain o f core volume fractions, a spectral signature very similar to the observed 13 mu m feature. However, the necessity to assume rather artificial distrib ution functions of the core volume fractions makes this scenario improbable . v) We consider MgAl2O4 (spinel) to be the most promising candidate for th e carrier of the 13 mu m feature since its emissivity peaks are located at 12.95 mu m and at 16.8 mu m for spherical particles and since at both posit ions features are present in most of the spectra of our sample. In view of the relevance of this substance for circumstellar shells, laboratory spectr a of spinel particles embedded in KBr are also presented.