New views of Betelgeuse: multi-wavelength surface imaging and implicationsfor models of hotspot generation

We report contemporaneous multi-wavelength interferometric imaging of the r ed supergiant star Betelgeuse (alpha Orionis), using the Cambridge Optical Aperture Synthesis Telescope (COAST) and the William Herschel Telescope (WH T), at wavelengths of 700, 905 and 1290 nm. We find a strong variation in t he apparent symmetry of the stellar brightness distribution as a function o f wavelength. At 700 nm the star is highly asymmetric, and can be modelled as the superposition of three bright spots on a strongly limb-darkened disc . However, at 905 nm only a single low-contrast feature is visible and at 1 290 nm the star presents a featureless symmetric disc. The change in spot c ontrast with wavelength is consistent with a model in which the bright spot s represent unobscured areas of elevated temperature, owing perhaps to conv ection, on a stellar disc that itself has a different appearance, i.e. geom etrical extent and limb-darkening profile, at different wavelengths. The fe atureless centre-to-limb brightness profile seen at 1290 nm is consistent w ith this model and suggests that future interferometric monitoring of the s tar to quantify the size changes associated with radial velocity variations should be performed at similar wavelengths in the near-infrared.