On the Use of Blanketed Atmospheres as Boundary Conditions for Stellar Evolutionary Models

Abstract

Stellar models have been computed for stars having [ Fe/H ] = 0.0 (assuming both the Grevesse & Sauval and Asplund et al. heavy-element mixtures) and –2.0 to determine the effects on the predicted Teff scale of using boundary conditions derived from the latest MARCS model atmospheres. The latter were fitted in a fully consistent way to the interior models at the photosphere and at τ = 100: the resultant evolutionary sequences on the H-R diagram were found to be nearly independent of the chosen fitting point. Tracks were also computed in which the pressure at T = Teff was obtained by integrating the hydrostatic equation together with either the classical gray T(τ , Teff) relation or that derived by Krishna Swamy from an empirical solar atmosphere. Due to the effects of differences in the solar-calibrated values of the mixing-length parameter, αMLT, very similar tracks were obtained for the different treatments of the atmosphere, except at solar abundances, where the models based on the Krishna Swamy T(τ , Teff) relationship predicted ~150 K hotter giant branches than the others, in good agreement with the inferred temperatures of giants in the open cluster M67 from recent (V − K) -Teff relations. Tracks that used new ``scaled solar, differentially corrected'' MARCS atmospheres were found to agree well with those that employed the Krishna Swamy T(τ , Teff) relationship, independently of the assumed metal abundance. (Gray atmospheres are quite different from MARCS models.) Fits of isochrones for [ Fe/H ] = − 2.0 to the CMD of the globular cluster M68, as well as the possibility that αMLT varies with stellar parameters, are also discussed.