The surface structure and limb-darkening profile of Betelgeuse

We present the first resolved image of a star observed with a separated-element optical aperture synthesis telescope. Observations of the M2 Iab supergiant Betelgeuse (alpha Orionis) were made at 830nm with the Cambridge Optical Aperture Synthesis Telescope (COAST) during 1995 October. Unusually, the source intensity distribution was highly symmetric at the time of observation - we find no evidence for the brightness asymmetries observed in earlier studies. Our data indicate a strongly limb-darkened but otherwise uniform and circularly symmetric disc, with no compact features contributing any more than 4 per cent of the total flux from the source. The brightness profile is characterized by a very flat core and a Gaussian-like tail, and cannot be represented satisfactorily by a conventional low-order Taylor expansion. A two-parameter Gauss-Hermite polynomial expansion provides a good description for the centre-to-limb brightness distribution.