Temporal Variations of Midinfrared Spectra in Late‐Type Stars

New multi-epoch, mid-infrared (8-13 microns) spectrophotometric observationsare presented for 30~late-type stars. The observations were collected over afour year period (1994-1997), permitting an investigation of the mid-infraredspectral shape as a function of the pulsation cycle (typically 1-2 years). Thespectra of stars with little excess infrared emission and those withcarbon-rich dust show the least spectral variability, while stars with evidencefor dusty, oxygen-rich envelopes are most likely to show discernible variationsin their spectral profile. Most significantly, a large fraction of variablestars with strong 9.7 micron emission features show clear spectral profilechanges which repeat from one cycle to the next. The significant sharpening ofthe silicate feature near maximum light can not be fully explained by heatingand cooling of the circumstellar dust shell during the pulsational cycle,suggesting that the dust optical properties themselves must also be varying. Inaddition, the appearance of a narrow emission feature near the silicate peakfor a few stars may require the production of especially ``pure'' silicate dustnear maximum light. The general narrowing of the silicate feature observed mayreflect the evolution of the pre-existing dirty grains whose surface impuritieshave been evaporated off when the grain temperature rises preceding maximumlight. An improved theory of dust formation which can explain the observedchanges in the grain properties around a single, pulsating star may lead to adefinitive explanation for the diversity of silicate emission profiles observedamongst oxygen-rich, late-type stars.Comment: 28 pages (2 tables & 14 pages of figures). Uses aaspp4 & flushrt macros. Accepted by the Astrophysical Journa