The Keck Aperture Masking Experiment: dust-enshrouded red giants

While the importance of dusty asymptotic giant branch (AGB) stars to galactic chemical enrichment is widely recognized, 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 20 dust-enshrouded AGB stars as part of the Keck Aperture Masking Experiment, resolving all of them in multiple nearinfrared bands between 1.5 and 3.1 μm. We find 45 per cent of the targets to show measurable elongations that, when correcting for the greater distances of the targets, would correspond to significantly asymmetric dust shells at par with the well-known cases of IRC+ 10216 or CIT 6. Using radiative transfer models, we find the sublimation temperature of Tsub (silicates) = 1130 ± 90 K and Tsub (amorphous carbon) = 1170 ± 60 K, both somewhat lower than expected from laboratory measurements and vastly below temperatures inferred from the inner edge of young stellar objects discs. 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 (τ2.2 μm > 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.