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We present recent studies using the near-infrared instrument AMBER of the VLTInterferometer (VLTI) to investigate the structure and shaping processes withinthe extended atmosphere of AGB stars. Spectrally resolved near-infrared AMBERobservations of the Mira variable S Ori have revealed wavelength-dependentapparent angular sizes. These data were successfully compared to dynamic modelatmospheres, which predict wavelength-dependent radii because of geometricallyextended molecular layers. Most recently, AMBER closure phase measurements ofseveral AGB stars have also revealed wavelength-dependent deviations from 0/180deg., indicating deviations from point symmetry. The variation of closure phasewith wavelength indicates a complex non-spherical stratification of theextended atmosphere, and may reveal whether observed asymmetries are locatednear the photosphere or in the outer molecular layers. Concurrent observationsof SiO masers located within the extended molecular layers provide us withadditional information on the morphology, conditions, and kinematics of thisshell. These observations promise to provide us with new important insightsinto the shaping processes at work during the AGB phase. With improved imagingcapabilities at the VLTI, we expect to extend the successful story of imagingstudies of planetary nebulae to the photosphere and extended outer atmosphereof AGB stars.

Structure and shaping processes within the extended atmospheres of AGB stars