Population II Dwarfs

Population II stellar evolution models have now been calculated for various metallicities. They are compared to observations of globular clusters and of field Pop II stars. Atomic diffusion affects the surface composition through separation occurring below the surface convection zone. It requires that the surface lithium abundance must have been reduced from the cosmological one. In the lowest metallicity clusters, in the absence of a process competing with diffusion, one would expect overabundances of metals and in particular of iron. Comparison to observed surface abundances suggests that turbulence is involved in modifying the efficiency of atomic diffusion. Models calculated with mass loss are less compatible with observations than those with turbulence. The age of globular clusters is reduced by about 2 Gyr which makes it compatible with recent determinations of the age of the universe.