100-yr mass-loss modulations on the asymptotic giant branch

We analyse the differences in infrared circumstellar dust emission between oxygen-rich Mira and non-Mira stars, and find that they are statistically s ignificant. In particular, we find that these stars segregate in the K-[12] versus [12]-[25] colour-colour diagram, and have distinct properties of th e IRAS LRS spectra, including the peak position of the silicate emission fe ature. We show that the infrared emission from the majority of non-Mira sta rs cannot be explained within the context of standard steady-state outflow models. The models can be altered to fit the data for non-Mira stars by postulating non-standard optical properties for silicate grains, or by assuming that t he dust temperature at the inner envelope radius is significantly lower (30 0-400 K) than typical silicate grain condensation temperatures (800-1000 K) . We argue that the latter is more probable and provide detailed model fits to the IRAS LRS spectra for 342 stars. These fits imply that two-thirds of non-Mira stars and one-third of Mira stars do not have hot dust (> 500 K) in their envelopes. The absence of hot dust can be interpreted as a recent (similar to 100 yr) decrease in the mass-loss rate. The distribution of best-fitting model para meters agrees with this interpretation and strongly suggests that the mass loss resumes on similar time-scales. Such a possibility appears to be suppo rted by a number of spatially resolved observations (e.g. recent Hubble Spa ce Telescope images of the multiple shells in the Egg Nebula) and is consis tent with new dynamical models for mass loss on the asymptotic giant branch .