VLTI/MIDI observations of the circumstellar shell of an AGB star: W Hydrae

W Hydrae is a prototype AGB star with a dusty circumstellar shell. The shell is marginally resolved at ground-based subarcsecond resolution; the unresolved dust condensation radius is expected to be of order 30-50 mas. Since the dusty circumstellar shell is brightest in the mid-IR, the high spatial resolution required at midinfrared wavelengths can currently only be achieved by using MIDI on the VLTI. We therefore outline a possible observing programme for the VLTI, using MIDI at 10 pm. In a single night of short-baseline VLTI observations with two Auxiliary Telescopes, we can achieve sufficient (u,v) plane coverage to resolve the shell geometry of W Hya, which will provide strong constraints on the mass loss processes in late-type stars. We expect to either obtain an estimate of the dust condensation temperature, or evidence for interrupted mass loss.