Multi-wavelength interferometry of evolved stars using VLTI and VLBA

We report on our project of coordinated VLTI/VLBA observations of the atmospheres and circumstellar environments of evolved stars. We illustrate in general the potential of interferometric measurements to study stellar atmospheres and envelopes, and demonstrate in particular the advantages of a coordinated multiwavelength approach including near/mid-infrared as well as radio interferometry. We have so far made use of VLTI observations of the nearand mid-infrared stellar sizes and of concurrent VLBA observations of the SiO maser emission. To date, this project includes studies of the Mira stars SOri and RRAql as well as of the supergiant AHSco. These sources all show strong silicate emission features in their mid-infrared spectra. In addition, they each have relatively strong SiO maser emission. The results from our first epochs of SOri measurements have recently been published [5] and the main results are reviewed here. The SOri maser ring is found to lie at a mean distance of about 2 stellar radii, a result that is virtually free of the usual uncertainty inherent in comparing observations of variable stars widely separated in time and stellar phase. We discuss the status of our more recent SOri, RRAql, and AHSco observations, and present an outlook on the continuation of our project.