Infrared optical properties of spinels - A study of the carrier of the 13,17 and 32 mu m emission features observed in ISO-SWS spectra of oxygen-rich AGB stars

In a previous paper, we have proposed magnesium aluminium spinel to be the carrier of the 13 and 17 mum band features observed in the ISO spectra of s ome red giants. The IR optical properties of spinel strongly depend on its chemistry and its internal structure. To study the dependence of spinel's I R-spectra on its aluminium content, we have synthesized a number of crystal s with different Al/Mg-ratios. Additionally, we performed an annealing expe riment to investigate the phase transition between ordered and partially di sordered spinel taking place at about 1200 K. We derived sets of optical co nstants of our natural, annealed and synthetic spinels in order to calculat e the absorption efficiencies of small (sub-mum-sized) spherical particles. Thereby, it turned out that natural as well as near-stoichiometric synthet ic spinel can indeed be considered as a suitable candidate for the carrier of the 13 mum feature observed in the spectra of some oxygen rich circumste llar shells. To illustrate this, we reinvestigated the mean profile of the residual dust emission in the 12-18 mum wavelength range (i.e., in the so-c alled trough region between the two silicate bands). The reality of the emi ssion feature at 16.8 mum is confirmed by our new investigation. We demonst rated that it is not an instrumental artifact since it is not present in IS O spectra of K-stars. In the course of our laboratory work, we found a thir d prominent emissivity maximum of spinel at 32 mum. This feature could also be detected in the spectra of the brightest 13 mum band emission sources; we derived its mean band profile, too. The new sets of optical constants he re presented have been made available for public access in the electronic d atabase http://www.astro.uni-jena.de.