Models for Solar Abundance Stars with Gravitational Settling and Radiative Accelerations: Application to M67 and NGC 188

Evolutionary models taking into account radiative accelerations, thermaldiffusion, and gravitational settling for 28 elements, including all thosecontributing to OPAL stellar opacities, have been calculated for solarmetallicity stars of 0.5 to 1.4 solar masses. The Sun has been used tocalibrate the models. Isochrones are fitted to the observed color-magnitudediagrams (CMDs) of M67 and NGC188, and ages of 3.7 and 6.4 Gyr are respectivelydetermined. Convective core overshooting is not required to match the turnoffmorphology of either cluster, including the luminosity of the gap in M67,because central convective cores are larger when diffusive processes aretreated. This is due mainly to the enhanced helium and metal abundances in thecentral regions of such models. The observation of solar metallicity openclusters with ages in the range 4.8--5.7Gyr would further test the calculationsof atomic diffusion in central stellar regions: according to non-diffusiveisochrones, clusters should not have gaps near their main-sequence turnoffs ifthey are older than ~4.8Gyr, whereas diffusive isochrones predict that gapsshould persist up to ages of ~5.7Gyr. Surface abundance isochrones are alsocalculated. In the case of M67 and NGC188, surface abundance variations areexpected to be small. Abundance differences between stars of very similar Teffare expected close to the turnoff, especially for elements between P and Ca.Moreover, in comparison with the results obtained for giants, small generalizedunderabundances are expected in main-sequence stars. The lithium to berylliumratio is discussed briefly and compared to observations.Comment: 34 pages, 15 figures, Accepted by the Astrophysical Journa