Multi-wavelength polarization studies of AGB star envelopes — a progress report

The circumstellar envelopes (CSE) of asymptotic giant branch (AGB) stars provide natural laboratories for the study of, especially, dust processes in chemically homogeneous environments. Because they progress from oxygen dominated (with silicate and metal oxide grains) to carbon dominated (with amorphous carbon and SiC dust) separate dust components can be isolated and studied.

We have observed and analyzed optical, far infrared (SOFIA/HAWC+) and sub-mm wave (JCMT/POL-2) polarization data for IRC+10216 ([C]/[O]>1), IK Tau ([C]/[O]<1) and W Aql (([C]/[O]≈1; JCMT/POL-2 only) to probe the dust physics, especially grain alignment, under these differing environments. We find very distinct differences between silicate and carbon dominated environments, but also that the general CSE environment, including grain charging, plays an important role in the results. The broad wavelength range allows different aspects of the grain alignment, dust characteristics and environmental characteristics to be studied. We will present a progress report of this work.