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 \mu m and 3.1 \mu m. 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 CIT 6 .
Using radiative transfer models , we find the sublimation temperature of T _ { sub } ( silicates ) = 1130 \pm 90 K and T _ { sub } ( amorphous carbon ) = 1170 \pm 60 K , 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 _ { 2.2 \mu { m } } > 2 ) , 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 .