Diffusion and Mixing of Lithium and Helium in Population II Dwarfs

Evolutionary models are calculated for low-mass metal-poor stars that simultaneously include the gravitational settling of He and Li. These calculations use the best available atomic diffusion coefficients and consistently follow diffusion throughout the stellar interior. It is found that in models of low-mass metal-poor stars, central settling of He reduces the age for a given turn-off luminosity by about 12-15 percent. In light of the uncertainties in the distance scale and O abundances of globular clusters, the inclusion of gravitational settling may give globular cluster ages as small as 12 Gyr. The calculations also suggest that the observed 'plateau' Li abundance in halo stars is at least a factor of 1.7 below the initial stellar abundance. Models that include turbulence cannot eliminate gravitational settling without destroying more Li than is consistent with the observations. The mere presence of Li in halo stars implies that substantial gravitational settling of He has occurred in these same stars. A significant reduction of the ages of globular clusters due to the effects of diffusion seems unavoidable.