Solution to the problem of the surface gravity distribution of cool DA white dwarfs from improved 3D model atmospheres

The surface gravities of cool (T-eff < 13 000 K) hydrogen-atmosphere DA white dwarfs, determined from spectroscopic analyses, are found to be significantly higher than the canonical value of log g similar to 8 expected for these stars. It was recently concluded that a problem with the treatment of convective energy transport within the framework of the mixing-length theory was the most plausible explanation for this high-log g problem. We pursue the investigation of this discrepancy by computing model spectra of cool convective white dwarfs from a small sequence (11 300 K < T-eff < 12 800 K) of 3D hydrodynamical model atmospheres, which feature a sophisticated treatment of convection and radiative transfer. Our approach is to proceed with a differential analysis between 3D and standard 1D models. We find that the 3D spectra predict significantly lower surface gravities, with corrections of the right amplitude as a function of effective temperature to obtain values of log g similar to 8 on average. We conclude that the surface gravity distribution of cool convective DA white dwarfs is much closer to that of hotter radiative objects when using, for the treatment of the convection, 3D models instead of the mixing-length framework.