A semitheory for semiconvection

Semiconvection is the name given to a situation that arises in the evolution of large-mass main-sequence stars and lower-mass horizontal-branch stars, in which a layer forms where almost all of the heat flux is transported by radiation, but where slow convection is required to redistribute a stably stratified solute (e.g. helium). For a general discussion on semiconvection, see Spiegel (1969). The main problem is to determine the correct relationship between the solute distribution and the temperature gradient in the inhomogeneous layer. Ledoux (1947) and Schwarzschild and Harm (1958) have proposed two very different prescriptions. Since theory and experiment indicate that the Schwarzschild-Harm (SH) prescription is correct for the onset of convection (in the form of overstability), Spiegel (1969) has proposed that SH are correct.