Convective overshooting in main sequence models

The estimate of the overshooting made here is based on the familiar concept of a convective element, which is suggested by the mixing-length formalism. The overshooting is discussed in terms of a rising convective element which reaches the formal boundary of the core, and continues to rise until buoyant deceleration stops it. Included in the calculation is the energy flux (which is negative) carried by the convective element. The importance of including this cooling by convective elements has been demonstrated by Shaviv and Salpeter (1973). To simplify the calculation I shall consider only a 'typical' element which crosses the boundary with zero temperature excess. This description suggests that overshooting is not an episodic phenomenon due to occasionally energetic elements, but is rather a feature of the steady pattern of convection. In addition the extent of the motions will be increased by their ability to cool the medium, since this will steepen the temperature gradient (bringing it nearer to the adiabatic), thus reducing the decelerating effects. (author)