  We analyze the differences in infrared circumstellar dust emission between
oxygen rich Mira and non-Mira stars, and find that they are statistically
significant. In particular, we find that these stars segregate in the K-[12]
vs. [12]-[25] color-color diagram, and have distinct properties of the IRAS LRS
spectra, including the peak position of the silicate emission feature. We show
that the infrared emission from the majority of non-Mira stars 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 the
dust temperature at the inner envelope radius is significantly lower (300-400
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 2/3 of non-Mira stars and 1/3
of Mira stars do not have hot dust ($>$ 500 K) in their envelopes. The absence
of hot dust can be interpreted as a recent (order of 100 yr) decrease in the
mass-loss rate. The distribution of best-fit model parameters agrees with this
interpretation and strongly suggests that the mass loss resumes on similar time
scales. Such a possibility appears to be supported by a number of spatially
resolved observations (e.g. recent HST images of the multiple shells in the Egg
Nebula) and is consistent with new dynamical models for mass loss on the
Asymptotic Giant Branch.
