The changing face of Betelgeuse

We describe a sequence of four optical interferometric observations of Betelgeuse, obtained using the non-redundant aperture mask method at the 4.2-m William Herschel Telescope on La Palma between 1994 November and 1995 January. The observations reveal complex asymmetries in the brightness distribution of the star, detected at a very high signal-to-noise ratio. Adequate modelling of the intensity structure requires at least three bright spots superposed on the stellar disc. Changes in the relative flux and positions of the spots are evident over the 8-week period spanning the observations. The brightness and maximum size of these features are consistent with convective surface hotspots. Simultaneous photometry of the integrated light of Betelgeuse shows a large and rapid dimming over the same period, which is not correlated with any change in the bright features. We deduce that, in this instance, the photometric variation did not result from localized convective activity associated with the spots. The interferometric data also show strong evidence for structure on a scale much larger than the photospheric diameter, which has not been present in previous observations over recent years. We attribute this new component to scattering from a newly formed dust halo, with a minimum angular diameter of approximately 0.3 arcsec. Since the brightness of this structure relative to the stellar disc did not change significantly throughout our observations, it is unlikely that the dimming of the star resulted from increased absorption by the dust halo. We conclude that the fading was due to a global expansion and cooling of the photosphere.