Heavy‐Element Diffusion in Metal‐poor Stars

Stellar evolution models which include the effect of helium and heavy elementdiffusion have been calculated for initial iron abundances of [Fe/H] = -2.3,-2.1, -1.9 and -1.7. These models were calculated for a large variety of massesand three separate mixing lengths, \alpha = 1.50, 1.75 and 2.00 (with \alpha =1.75 being the solar calibrated mixing length). The change in the surface ironabundance for stars of different masses was determined for the ages 11, 13 and15 Gyr. Iron settles out of the surface convection zone on the main sequence;this iron is dredged back up when the convection zone deepens on the giantbranch. In all cases, the surface [Fe/H] abundance in the turn-off stars was atleast 0.28 dex lower than the surface [Fe/H] abundance in giant-branch stars ofthe same age. However, recent high dispersion spectra of stars in the globularcluster NGC 6397 found that the turn-off and giant branch stars had identical(within a few percent) iron abundances of [Fe/H] =-2.03 (Gratton et al. 2001).These observations prove that heavy element diffusion must be inhibited in thesurface layers of metal-poor stars. When diffusion is inhibited in the outerlayers of a stellar model, the predicted temperatures of the models are similarto models evolved without diffusion, while the predicted lifetimes are similarto stars in which diffusion is not inhibited. Isochrones constructed from themodels in which diffusion is inhibited fall half-way between isochrones withoutdiffusion, and isochrones with full diffusion. As a result, absolute globularcluster ages which are based upon the absolute magnitude of the turn-off are 4%larger than ages inferred from full diffusion isochrones, and 4% smaller thanages inferred from no diffusion isochrones.Comment: to appear in ApJ; 11 page