THE AGE OF GLOBULAR-CLUSTERS IN LIGHT OF HIPPARCOS - RESOLVING THE AGE PROBLEM

We review five independent techniques that are used to set the distanc e scale to globular clusters, including subdwarf main-sequence fitting utilizing the recent Hipparcos parallax catalog. These data together all indicate that globular clusters are farther away than previously b elieved, implying a reduction in age estimates. We now adopt a best-fi t value M-epsilon (RR Lyrae stars) = 0.39 +/- 0.08 (statistical) at [F e/H] = - 1.9 with an additional uniform systematic uncertainty of (+ 0 .13)(- 0.18). This new distance scale estimate is combined with a deta iled numerical Monte Carlo study (previously reported by Chaboyer et a l.) designed to assess the uncertainty associated with the theoretical age-turnoff luminosity relationship in order to estimate both the abs olute age and uncertainty in age of the oldest globular clusters. Our best estimate for the mean age of the oldest globular clusters is now 11.5 +/- 1.3 Gyr, with a one-sided 95 % confidence level lower limit o f 9.5 Gyr. This represents a systematic shift of over 2 sigma compared to our earlier estimate, owing completely to the new distance scale-a shift which we emphasize results not only from the Hipparcos data. Th is now provides a lower limit on the age of the universe that is consi stent with either an open universe or with a flat matter-dominated uni verse (the latter requiring H-0 less than or equal to 67 km s(-1) Mpc( -1)). Our new study also explicitly quantifies how remaining uncertain ties in the distance scale and stellar evolution models translate into uncertainties in the derived globular cluster ages. Simple formulae a re provided that can be used to update our age estimate as improved de terminations for various quantities become available. Formulae are als o provided that can be used to derive the age and its uncertainty for a globular cluster, given the absolute magnitude of the turnoff or the point on the subgiant branch 0.05 mag redder than the turnoff.