Hydrogen-helium adiabats for late-type stars

Abstract

For late-type main-sequence stars, the earlier models that consisted of convective cores and radiative envelopes have been superseded by models with radiative cores and convective envelopes, topped by an atmosphere in radiative equilibrium. But in the available models of the latter type, the representation of the convective zone has been rather schematic, in so far as the adiabats used would apply only if the gas were completely ionized. Accordingly, an expression for the adiabatic gradient of hydrogen-helium mixtures has been derived from thermodynamics. This derivation takes account of the interlocking ionization of hydrogen and helium and of its coupling with the association of hydrogen into molecules. Adiabats have been computed with three values of hydrogen to helium ratio (by number), 8/1, 16/1, and ∞. Moreover, an expression for the adiabatic gradient, in which the effect of pressure ionization has been included in an approximate way, has been given for the special case of pure hydrogen.