Radiation-hydrodynamics Simulations of Surface Convection in a Late M-dwarf

Based on detailed 2D and 3D numerical radiation hydrodynamicscalculations of time-dependent compressible convection, we have studiedthe dynamics and thermal structure of the convective surface layers of aprototypical late-type M-dwarf at T<SUB>eff</SUB>=2800 K, and log g=5.0with solar chemical composition. The thermal structure of the atmosphereis dominated by molecular absorption, the formation of dust grains isnot important for the chosen model parameters. In the multi dimensionalmodels the radiative transfer is treated by a multi-group (4 groups)approach which provides a simplified, nevertheless rather realistictreatment of the complex radiative energy transport. The equation ofstate includes the important contribution of H<SUB>2</SUB> moleculeformation. Our models predict a convective pattern at the surface of anlate M-dwarf qualitatively similar to solar granulation. Quantitatively,the convective turn-over timescale amounts to &approx; 100 s, a typicalhorizontal scale of convective cells to 80 km, and a relative intensitycontrast of the granular pattern to 1.1 %. The efficiency of convectiveenergy transport corresponds to an effective mixing-length parameterbetween 1.5 to 2.1 depending on the thermal property which isrepresented. The models provide mixing timescales due to atmosphericovershoot which can be extrapolated to lower effective temperatureswhere dust grains are present and mixed into optically thin layers.