Reality and the speckle imaging of stellar surfaces - II. the asymmetry of Alpha Orionis

Bispectrum image reconstructions of the red supergiant Alpha Orionis, based upon CCD observations made with the 4.2-m William Herschel Telescope in 1995 January, are presented, and the statistical significance of asymmetries across the stellar disc established at better than 99 per cent certainty. Asymmetry is present in at least two wavebands, although the statistical evidence is stronger in the continuum than within a TiO absorption band. The asymmetry may be interpreted as being due to the presence of a bright feature contributing some 15-20 per cent of the total observed disc brightness. Ambiguities present in simple limb-darkened model fits to the power spectrum are shown, and the allowed range of limbdarkening coefficients and disc diameters is given.