  While the importance of dusty asymptotic giant branch (AGB) stars to galactic
chemical enrichment is widely recognised, a sophisticated understanding of the
dust formation and wind-driving mechanisms has proven elusive due in part to
the difficulty in spatially-resolving the dust formation regions themselves. We
have observed twenty dust-enshrouded AGB stars as part of the Keck Aperture
Masking Experiment, resolving all of them in multiple near-infrared bands
between 1.5 microns and 3.1 microns. We find 45% of the targets to show
measurable elongations that, when correcting for the greater distances of the
targets, would correspond to significantly asymmetric dust shells on par with
the well-known cases of IRC+10216 or CIT6. Using radiative transfer models, we
find the sublimation temperature of 1130 +- 90 K and 1170 +- 60 K for silicates
and amorphous carbon respectively, both somewhat lower than expected from
laboratory measurements and vastly below temperatures inferred from the inner
edge of YSO disks. The fact that O-rich and C-rich dust types showed the same
sublimation temperature was surprising as well. For the most optically-thick
shells (tau > 2 at 2.2 microns), the temperature profile of the inner dust
shell is observed to change substantially, an effect we suggest could arise
when individual dust clumps become optically-thick at the highest mass-loss
rates.
