On the origin of the 19.5 μ m feature. Identifying circumstellar Mg-Fe-oxides

We report the detection of a broad, prominent 19.5 mu m dust emission feature in ISO-SWS spectra. It is especially conspicuous in the spectra of low mass-loss AGB stars belonging to the variability classes SRb and Lb with relatively high photospheric temperatures. The feature carrier is proposed to be Mg_0.1Fe_0.9O, a solid solution of MgO (periclase) and FeO (wustite). This dust species has cubic crystal symmetry like spinel, the carrier of the 13, 16.8 and 31.8 mu m spectral features, together with which the 19.5 mu m feature is being observed. A broad emission plateau between 11 and 15 mu m, which is attributable to amorphous Al₂O₃, is also detected together with the 19.5 mu m and the spinel features. As a consequence of our discovery, we postulate the existence of a distinct class of circumstellar shells, dominated by oxide and not by silicate dust. 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 UK) and with the participation of ISAS and NASA.