LITHIUM IN HALO STARS - CONSTRAINING THE EFFECTS OF HELIUM DIFFUSION ON GLOBULAR-CLUSTER AGES AND COSMOLOGY

It has been proposed that helium diffusion can reduce age estimates for globular clusters through its effect on the main-sequence lifetime as well as on the morphology of isochrones near the turnoff. Using stellar evolutionary models with diffusion, both ours and from other work, we show that observations of lithium in extreme halo stars provide crucial constraints on the magnitude of the effects of helium diffusion. The flatness (and/or possible slight increasing slope) of the observed Li-T(eff) relation (the Spite Li plateau) severely constrains diffusion Li isochrones, which tend to curve downward toward higher T(eff). Previously published age reductions of order 20%-30% seem to be ruled out by the Li flatness constraint from the current observations; however, age reductions of order 10% might be possible. Additional lithium observations are urgently required, particularly toward the hot edge of the plateau as well as beyond the cool edge, to define more securely the morphology of the Li-T(eff) trend, and thus to gauge more precisely the possible effects of helium diffusion.