Convective penetration into stellar radiation zones

Abstract

We recall the evidence for convective penetration exhibited by geophysical fluids, laboratory experiments and computer simulations. A simple model is deduced from these observations, which serves to establish a relation between the subadiabatic extent of a convective region and the velocity of the penetrating motions. Assuming that this velocity obeys the usual scaling of thermal convection, we find 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. We show 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.