The PI will continue a research program on mass loss processes in intermediate mass stars in the late stages of their evolution, with emphasis on the so-called asymptotic giant branch stars, and the subsequent stages where expansion leads to formation of planetary nebulae. The PI will endeavor to understand the physical processes in these evolved stars by which pulsation, shocks, non-equilibrium chemistry, and dust formation drive the stellar mass loss and determine the composition of the ejected gas and dust. The PI will use optical telescopes with a sensitive, wide-band spectropolarimeter to measure the optical polarization as a function of wavelength for a selection of evolved stars. Polarized light reflected off dust grains near such stars is a diagnostic of newly-formed dust very close to the star. Comparison of models and data should give uniquely powerful constraints on the location and properties of dust formed in the circumstellar environment of these stars. Furthermore the PI will obtain mm and submm observations of key molecular emission lines to constrain the spatial distribution of molecules in the circumstellar envelopes of the evolved stars. One key goal of this study will be to test whether the molecular abundances and excitation conditions imply that pulsation-driven shocks in the stellar atmosphere determine the chemical composition of the ejected matter. Additional ~1 arcsec resolution images of a small sample of bright nearby giant stars in light of selected molecular emission lines that are tracers for gas temperature and density will give information on the physical conditions in the small-scale structures detected previously by the PI in their circumstellar envelopes
Broader impact: The results are disseminated not only in the scientific literature, but also to the wider community via public lectures and displays, routinely provided by the Steward Observatory to our community, with an emphasis on making the science accessible to the interested public, and on stimulating further learning. This research has an important education function by engaging both undergraduate and graduate students, and by allowing and encouraging them to attend scientific meetings to present the results of the work in which they participated.
