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The atmospheres of cool stars are temporally and spatially inhomogeneous dueto the effects of convection.The influence of this inhomogeneity, referred toas granulation, on colours has never been investigated over a large range ofeffective temperatures and gravities. We aim to study, in a quantitative way,the impact of granulation on colours. We use the CIFIST grid of CO5BOLDhydrodynamical models to compute emerging fluxes. These in turn are used tocompute theoretical colours in the UBVRI,2MASS,Hipparcos,Gaia and SDSS systems.Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHDmodel computed for the same atmospheric parameters, which we used to define a"3D correction" that can be applied to colours computed from fluxes computedfrom any 1D model atmosphere code. The 3D corrections on colours are generallysmall, of the order of a few hundredths of a magnitude, yet they are far fromnegligible. We find that ignoring granulation effects can lead tounderestimation of Teff by up to 200K and overestimation of gravity by up to0.5dex, when using colours as diagnostics. We have identified a majorshortcoming in how scattering is treated in the current version of the CIFISTgrid, which could lead to offsets of the order 0.01mag, especially for coloursinvolving blue and UV bands.We have investigated the Gaia and Hipparcosphotometric systems and found that the (G-H_p),(BP-RP) diagram is immune to theeffects of granulation. In addition, we point to the potential of the RVSphotometry as a metallicity diagnostic. Our investigation shows that theeffects of granulation should not be neglected if one wants to use colours asdiagnostics of the stellar parameters of F,G,K stars. A limitation is thatscattering is treated as true absorption in our current computations, thus our3D corrections are likely an upper limit to the true effect. (Abridged)

astronomy and astrophysics

Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the  effects of stellar granulation on photometric colours. I. Grids of 3D  corrections in the UBVRI, 2MASS, Hipparcos, Gaia, and SDSS systems