(Proposal ID) S16B-057

(PI) Guyon, Olivier

(Proposal Title)
High resolution imaging of limb-polarization and mass-loss in evolved stars

(Abstract)
Mass loss from stars in the last stages of life governs their ultimate destiny 
as a supernovae or PNe, and is crucial for galactic chemical enrichment. The 
mechanisms driving the mass loss remain poorly understood; to accelerate a 
dust-driven wind requires grains to form too close to the star, while aspheric 
and clumpy envelopes require an additional mechanism, with multiple models 
proposed. Moreover, the stellar atmospheres themselves -- integral to the mass-
loss process -- are also poorly understood, and anticipated limb polarization 
effects may both give insight to the atmosphere itself but also have a 
confounding effect on polarization studies of the mass-loss shell. We propose 
to observe a set of RSG and AGB stars, as well as resolved K stars, using 
coronagraphic NIR imaging (with SCExAO/HiCIAO) and - simultaneously - visible, 
polarimetric interferometry using VAMPIRES. Providing maximum telescope 
productivity, HiCIAO images will reveal the structure - and hence mass-loss 
history - in the envelopes of the objects, while VAMPIRES will provide 
diffraction-limited observations of the innermost circumstellar regions where 
the dust formation process takes place, constraining dust grain properties 
using polarization. At the same time, the limb polarization of the stellar 
atmosphere can be observed.