Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours

Aims. We studied the influence of convection on the spectral energy distributions (SEDs), photometric magnitudes, and colour indices of different types of stars across the H-R diagram. Methods. The 3D hydrodynamical (COBOLD)-B-5, averaged < 3D >, and 1D hydrostatic LHD model atmospheres were used to compute SEDs of stars on the main sequence (MS), main sequence turn-off (TO), subgiant branch (SGB), and red giant branch (RGB), in each case at two different effective temperatures and two metallicities, [M/H] = 0.0 and -2.0. Using the obtained SEDs, we calculated photometric magnitudes and colour indices in the broad-band Johnson-Cousins UBVRI and 2MASS JHK(s), and the medium-band Stromgren uvby photometric systems. Results. The 3D-1D differences in photometric magnitudes and colour indices are small in both photometric systems and typically do not exceed +/- 0.03 mag. Only in the case of the coolest giants located on the upper RGB are the differences in the U and u bands able reach approximate to -0.2 mag at [M/H] = 0.0 and approximate to -0.1 mag at [M/H] = 2.0. Generally, the 3D-1D differences are largest in the blue-UV part of the spectrum and decrease towards longer wavelengths. They are also sensitive to the effective temperature and are significantly smaller in hotter stars. Metallicity also plays a role and leads to slightly larger 3D-1D differences at [M/H] = 0.0. All these patterns are caused by a complex interplay between the radiation field, opacities, and horizontal temperature fluctuations that occur due to convective motions in stellar atmospheres. Although small, the 3D-1D differences in the magnitudes and colour indices are nevertheless comparable to or larger than typical photometric uncertainties and may therefore cause non-negligible systematic differences in the estimated effective temperatures.