THE EFFECT OF HELIUM DIFFUSION ON THE AGES OF GLOBULAR-CLUSTERS

We have calculated evolutionary tracks for halo stars and constructed isochrones which included the effects of microscopic diffusion of helium. The isochrones were fitted to a metal poor (M92) and a moderately metal rich (NGC 288) globular cluster using an updated version of the Revised Yale Isochrone color calibration. Ages of the two clusters were also determined using the difference between the turnoff magnitude and horizontal branch magnitude, DELTA-V(TO - HB), and the difference in color between the main-sequence turnoff and lower giant branch, DELTA(B - V). Considering all methods and constraints, our best estimate is that diffusion reduces the derived ages of M92 and NGC 288 by 0.5-1 Gyr. The maximum age reduction that diffusion could cause is 3 Gyr. Age estimates including diffusion indicate that M92 is 16 +/- 2 Gyr old, and that M92 is approximately 3 Gyr older than NGC 288, assuming that the clusters have the same [O/Fe] of +0.4. The observations of Li abundances can be used as a tracer of the degree of helium diffusion. Both our standard and our diffusive models produce acceptable fits to observations of Li in halo stars. Acceptably fitting diffusive models have a primordial Li abundance (2.24) that is only 0.09 dex higher than that of the standard models.