Magnesium-aluminium oxide (spinel)

Magnesium-aluminium spinel (MgAl$_2$O$_4$) has been considered as a primary condensate in the outflows of oxygen-rich AGB stars and as a potential carrier of the 13 $\mu$m emission band observed in the spectra of these stars [11]. Therefore, in the Jena laboratory, a systematic study of the infrared properties of Mg-Al oxides of both synthetic and natural origin was performed in order to derive the optical constants of these materials. This led to the discovery of two accompanying features in the astronomical spectra at larger wavelengths, thereby strongly supporting the idea of spinel condensates in AGB star outflows (see Fig. , [12]). Recently, the experiments have been extended in the direction of Ca-Al oxide minerals [13] and condensation studies of oxide grains in low-presure oxygen-rich atmospheres.

Figure: Left panel: Imaginary part of the refractive index for synthetic and natural magnesium-aluminium spinels. Right panel: Calculated normalized absorption spectra for small particles composed of natural spinel (smooth solid line) and synthetic MgAl$_2$O$_4$ (dotted line) in comparison to the band profile of the newly discovered 32 $\mu$m feature [12]