A non-detection of red supergiant convection in Gaia

Abstract

Large-scale surface convection on red supergiants (RSGs) can lead to shifts in the photocentre of the star that might be measured by Gaia and used as a new probe of the surface dynamics of these rare but important stars. Unlike brightness variations, photocentre motions would provide information on the physical scale of the convective cells. The signal would be that RSGs show an excess astrometric noise at the level of a few per cent of the stellar radius. Unfortunately, we find that the excess astrometric noise level of Gaia EDR3 is roughly an order of magnitude too large to detect the predicted motions and that RSGs have excess astrometric noise indistinguishable from other stars of similar magnitude, colour, or parallax. The typical excess astrometric noise steadily decreases with G magnitude (for G < 11 mag), so it is crucial to compare stars of similar brightness.