Convective penetration in stellar interiors.

Evidence for convective penetration from a variety of sources is used to deduce a simple model which establishes a relation between the subadiabatic extent of a convective region and the velocity of the penetrating motions. It is found that the subadiabatic penetration at the bottom of a convective envelope is of the order of a pressure scale height, and that above a convective core it amounts to a substantial fraction of the core radius. It is shown that the requirements for nearly adiabatic penetration are met deep enough in a stellar interior and that the departures from adiabaticity are confined to a thin boundary layer. This allows the use of Roxburgh's integral constraint to predict the actual size of a convective core.