2000ApJ...531..917M

We have obtained N-band spectra for 18 M Mira variables with the Mid-Infrared Camera and Spectrometer (MICS) to investigate the dust materials formed around M Mira variables. The observed spectra show a wide variety; 15 of them show a strong feature at 10 µm, and eight and seven stars show 11 and 13 µm bumps, respectively. We have made an analysis of oxygen-rich dust spectra by using two representative spectra empirically derived. One is the 10 µm feature, and the other is the 11 µm feature. Linear combinations of the two dust spectra together with the stellar continuum can describe all the observed spectra quite successfully, indicating the presence of two kinds of dust materials around M Mira variables. The 10 µm feature is ascribed to silicate grains, while we propose to attribute the 11 µm feature to amorphous alumina grains. We found that crystalline silicates do not appear necessary to fit any component of the spectrum. We have compared the strengths of the dust emissions with the parameters of stellar variability. We found that the strength of the silicate emission increases as the light curve becomes asymmetric, while that of the alumina emission stays constant against the change of the asymmetry factor. This can be interpreted in terms of the differences in the dust-forming region.