VARIABLE DUST FEATURES IN INFRARED-SPECTRA OF AU CYGNI

We report the first clear evidence for variation in the infrared spect ral features due to silicates in the circumstellar shell of the oxygen -rich Mira variable AU Cygni, based on individual scans obtained with the IRAS low-resolution spectrometer, during the 1983 IRAS mission. Fo r the optically thin shell of AU Cyg, the contrast of the silicate fea ture is stronger near optical maximum and weaker near optical minimum. We propose that circumstellar dust has a significant population of sm all grains. This population may get enhanced near maximum, probably du e to evaporation of larger grains, increasing the amount of 10 mu m ba nd emission relative to continuum emission. The shape of the emission feature does not vary measurably from maximum to minimum brightness.