Photocentric variability of red supergiant stars and consequences on Gaia measurements

Abstract. Red supergiant stars are characterized by large convection-related surface structures that causesurface inhomogeneities and shock waves. We explore the impact of granulation on the photocentric motionusing 3D simulations of convection with CO5BOLD and post-processing radiative transfer code Optim3D tocompute spectra and intensity maps in the Gaia G band [325 { 1030 nm]. We found that the Gaia parallaxfor Betelgeuse-like supergiants are characterized by a systematic error of a few percents.Keywords: supergiants, astrometry, parallaxes, hydrodynamics 1 IntroductionThe aim of the Gaia mission is to determine high-precision astrometric parameters that, together with multi-band and multi-epoch photometric and spectrocopic data, will be used to reconstruct the formation, historyand evolution of the Galaxy.Red supergiant (RSG) stars are characterized by vigorous convection which imprints a pronounced granulationpattern on the stellar surface (Chiavassa et al. 2010). As a consequence, the granulation-related variability mustbe quanti ed in order to better characterize any resulting systematic error on the parallax determination.2 RHD simulations of red supergiant starsWe employed detailed radiation-hydrodynamics (RHD) simulation of RSGs (Freytag et al. 2002; Freytag &Hofner 2008). The model has a mass of 12 M , employs an equidistant numerical mesh with 235