Models for Solar Abundance Stars with Gravitational Settling and Radiative Accelerations: Application to M67 and NGC188

Abstract

Evolutionary models taking into account radiative accelerations, thermal diffusion, and gravitational settling for 28 elements, including all those contributing to OPAL stellar opacities, have been calculated for solar metallicity stars of 0.5 to 1.4 solar masses. The Sun has been used to calibrate the models. Isochrones are fitted to the observed color-magnitude diagrams (CMDs) of M67 and NGC188, and ages of 3.7 and 6.4 Gyr are respectively determined. Convective core overshooting is not required to match the turnoff morphology of either cluster, including the luminosity of the gap in M67, because central convective cores are larger when diffusive processes are treated. This is due mainly to the enhanced helium and metal abundances in the central regions of such models. The observation of solar metallicity open clusters with ages in the range 4.8--5.7Gyr would further test the calculations of atomic diffusion in central stellar regions: according to non-diffusive isochrones, clusters should not have gaps near their main-sequence turnoffs if they are older than ~4.8Gyr, whereas diffusive isochrones predict that gaps should persist up to ages of ~5.7Gyr. Surface abundance isochrones are also calculated. In the case of M67 and NGC188, surface abundance variations are expected to be small. Abundance differences between stars of very similar Teff are expected close to the turnoff, especially for elements between P and Ca. Moreover, in comparison with the results obtained for giants, small generalized underabundances are expected in main-sequence stars. The lithium to beryllium ratio is discussed briefly and compared to observations.