## 2. Overshooting as a diffusive processIn a seminal paper Freytag et al. (1996) have investigated the role of overshoot from the lower boundary of outer stellar convective layers. Based on two-dimensional hydrodynamical models they showed convincingly how the dynamics of turbulent convection is governed by the fast narrow downdrafts, which give rise to overshooting beyond the formal convective boundary. The simulations showed that the assumption of a finite overshooting distance does not represent the situation appropriately. Rather, the velocity field of the downward motions extends beyond the region with significant convective flux and declines exponentially. The scale length is correlated with the pressure scale height, but, and this is an important point, depends on the particular stellar parameters. Although the velocities decline rapidly, they are still of order of typical diffusion speeds, such that convective motions can counteract the sedimentation process. The temperature structure below the formal convective boundary remains dominated by radiative diffusion, as is necessary for SSM, which already without overshooting reproduce the depth of the solar convective envelope extremely well. This is the important difference to other overshooting approaches, in which the temperature gradient below the formally stable boundary is changed as well. The complete convective envelope and the adjacent overshooting region fit into the simulation box only for hot stars (A-type and DA white dwarfs). From this, Freytag et al. (1996) derived a relation for the diffusion coefficient appropriate for the overshooting: Here, Since the solar convective envelope could not be simulated
completely, Freytag et al. (1996) could argue only qualitatively about
the effect on the photospheric Bl"ocker et al. (1998) have calculated solar models using the
qualitative results of Freytag et al. (1996) to investigate the effect
of such an overshooting approach on the solar © European Southern Observatory (ESO) 1999 Online publication: June 18, 1999 |