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Polychromatic interferometry of evolved stars

M. Wittkowski with D. A. Boboltz (USNO), T. Driebe, K. Ohnaka (MPIfR),...

The evolution of cool luminous stars, including Mira variables, is accompanied by significant mass-loss to the circumstellar environment (CSE). The nature of this mass-loss process from evolved stars, and especially its connection with the pulsation mechanism in the case of Mira variable stars, is not well understood.
Furthermore, one of the basic unknowns in the study of late-type stars is the mechanism by which spherically symmetric stars on the asymptotic giant branch (AGB) evolve to form axisymmetric planetary nebulae. Coordinated multi-wavelength studies (near-infrared, mid-infrared, radio) of the stellar surface (photosphere) and the CSE at different distances from the stellar photosphere and -because of the variabbility- obtained at corresponding cycle+phase values of the stellar variability curve are best suited to improve our general understanding of the atmosphere, the CSE, the mass-loss process, and ultimately of the evolution of AGB stars toward planetary nebulae. The figure above shows a schematic view of a Mira variable star, indicating the different regions that can be probed by different techniques/wavelength ranges (MIDI, AMBER, VLBA, MERLIN).