Infrared optical properties of spinels -: A study of the carrier of the 13, 17 and 32 μm 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 some red giants. The IR optical properties of spinel strongly depend on its chemistry and its internal structure. To study the dependence of spinel's IR-spectra on its aluminium content, we have synthesized a number of crystals with different Al/Mg-ratios. Additionally, we performed an annealing experiment to investigate the phase transition between ordered and partially disordered spinel taking place at about 1200 K. We derived sets of optical constants of our natural, annealed and synthetic spinels in order to calculate the absorption efficiencies of small (sub-mum-sized) spherical particles. Thereby, it turned out that natural as well as near-stoichiometric synthetic 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 circumstellar 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-called trough region between the two silicate bands). The reality of the emission feature at 16.8 mum is confirmed by our new investigation. We demonstrated that it is not an instrumental artifact since it is not present in ISO spectra of K-stars. In the course of our laboratory work, we found a third 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 here presented have been made available for public access in the electronic database http://www.astro.uni-jena.de.