The nonspherical shape of Betelgeuse in the mid-infrared

Three-telescope interferometric observations from the Infrared Spatial Interferometer (ISI) are reported at spatial frequencies that resolve the size and shape of the star Betelgeuse (alpha Ori) at a wavelength of 11.15 mu m with a bandwidth of 0.18 cm(-1). The data include closure phase measurements, the first such measurements of a stellar disk in the mid- infrared. The data indicate a clear asymmetry of the stellar surface at these wavelengths. Mechanisms for generating the observed asymmetry via an elliptical shape or a hot spot near the southern limb of the star are discussed.