DYNAMICS of TURBULENT CONVECTION and CONVECTIVE OVERSHOOT in A MODERATE-MASS STAR

We present results of realistic three-dimensional (3D) radiative hydrodynamic simulations of the outer layers of a moderate-mass star (1.47 M _o), including the full convection zone, the overshoot region, and the top layers of the radiative zone. The simulation results show that the surface granulation has a broad range of scales, from 2 to 12 Mm, and that large granules are organized in well-defined clusters, consisting of several granules. Comparison of the mean structure profiles from 3D simulations with the corresponding one-dimensional (1D) standard stellar model shows an increase of the stellar radius by ∼800 km, as well as significant changes in the thermodynamic structure and turbulent properties of the ionization zones. Convective downdrafts in the intergranular lanes between granulation clusters reach speeds of more than 20 km s^-1, penetrate through the whole convection zone, hit the radiative zone, and form an 8 Mm thick overshoot layer. Contrary to semi-empirical overshooting models, our results show that the 3D dynamic overshoot region consists of two layers: a nearly adiabatic extension of the convection zone and a deeper layer of enhanced subadiabatic stratification. This layer is formed because of heating caused by the braking of the overshooting convective plumes. This effect has to be taken into account in stellar modeling and the interpretation of asteroseismology data. In particular, we demonstrate that the deviations of the mean structure of the 3D model from the 1D standard model of the same mass and composition are qualitatively similar to the deviations for the Sun found by helioseismology.