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

Abstract

We have derived a mean profile of the 13 mu m emission feature from 11 ISO spectra of oxygen-rich AGB stars and present an overview of the mineral species that could account for it. Our results can be summarized as follows: i) Spherical particles of alpha -Al_2O_3 (corundum) have a sharp emissivity maximum peaking at 12.7 mu m. The difference of 0.3 mu m in the peak position compared to the mean observed band profile is a serious obstacle to assigning the 13 mu m dust feature to this dust species. ii) From a continuous distribution of ellipsoidal alpha -Al_2O_3-particles, a broad emissivity profile peaking 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 TiO_2 (rutile) is located at 13.5 mu m; its width is larger and its strength three times smaller than the emissivity maximum of corundum. iv) Core-mantle-grains composed of rutile and corundum or of corundum and amorphous olivine can both produce, in a certain domain of core volume fractions, a spectral signature very similar to the observed 13 mu m feature. However, the necessity to assume rather artificial distribution functions of the core volume fractions makes this scenario improbable. v) We consider MgAl_2O_4 (spinel) to be the most promising candidate for the 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 positions features are present in most of the spectra of our sample. In view of the relevance of this substance for circumstellar shells, laboratory spectra of spinel particles embedded in KBr are also presented. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.}