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Local-box hydrodynamical model atmospheres provide statistical informationabout a star's emergent radiation field which allows one to predict the levelof its granulation-related micro-variability. Space-based photometry is nowsufficiently accurate to test model predictions. We aim to model thephotometric granulation background of HD49933 as well as the Sun, and comparethe predictions to the measurements obtained by the COROT and SOHO satellitemissions. We construct hydrodynamical model atmospheres representing HD49933and the Sun, and use a previously developed scaling technique to obtain theobservable disk-integrated brightness fluctuations. We further performedexploratory magneto-hydrodynamical simulations to gauge the impact of smallscale magnetic fields on the synthetic light-curves. We find that thegranulation-related brightness fluctuations depend on metallicity. We obtain asatisfactory correspondence between prediction and observation for the Sun,validating our approach. For HD49933, we arrive at a significantover-estimation by a factor of two to three in total power. Locally generatedmagnetic fields are unlikely to be responsible, otherwise existing fields wouldneed to be rather strong to sufficiently suppress the granulation signal.Presently suggested updates on the fundamental stellar parameters do notimprove the correspondence; however, an ad-hoc increase of the HD49933 surfacegravity by about 0.2dex would eliminate most of the discrepancy. We diagnose apuzzling discrepancy between the predicted and observed granulation backgroundin HD49933, with only rather ad-hoc ideas for remedies at hand.

Hydrodynamical simulations of convection-related stellar micro-variability. II. The enigmatic granulation background of the COROT target HD49933