Temporal variations of midinfrared spectra in late-type stars

New multiepoch, midinfrared (8-13 mu m) spectrophotometric observations are presented for 30 late-type stars. The observations were collected over a 4 yr period (1994-1997), permitting an investigation of the midinfrared spectral shape as a function of the pulsation cycle (typically 1-2 yr). The spectra of stars with little excess infrared emission and those with carbon-rich dust show the least spectral variability, while stars with evidence for dusty, oxygen-rich envelopes are most likely to show discernible variations in their spectral profile. Most significantly, a large fraction of variable stars with strong 9.7 mu m emission. features shows clear spectral profile changes, which repeat from one cycle to the next. The significant sharpening of the silicate feature near maximum light cannot be fully explained by heating and cooling of the circumstellar dust shell during the pulsational cycle, suggesting that the dust optical properties themselves must also be varying. In addition, the appearance of a narrow emission feature near the silicate peak for a few stars may require the production of especially "pure" silicate dust near maximum light. The general narrowing of the silicate feature observed may reflect the evolution of the preexisting dirty grains whose surface impurities have been evaporated off when the grain temperature rises preceding maximum Light. An improved theory of dust formation that can explain the observed changes in the grain properties around a single, pulsating star may lead to a definitive explanation for the diversity of silicate emission profiles observed among oxygen-rich, late-type stars.