The Age Of Globular Clusters In Light Of Hipparcos: Resolving the Age Problem?

We review five independent techniques which are used to set the distance scale to globular clusters, giving most weight to subdwarf main sequence fitting utilizing Hipparcos parallax results. These data indicate that globular clusters are farther away than previously believed, implying a reduction in age estimates. This new distance scale estimate is combined with a detailed numerical Monte Carlo study designed to assess the uncertainty associated with the theoretical age-turnoff luminosity relationship in order to estimate both the absolute age and uncertainty in age of the oldest globular clusters. Our best estimate for the mean age of the oldest globular clusters is now $11.7\pm 1.4 $Gyr, with a one-sided, 95% confidence level lower limit of 9.6 Gyr. This now provides a lower limit on the age of the universe which is consistent with either an open universe, or a flat, matter dominated universe (the latter requiring $H_0 \le 66 \kmsmpc$). Simple formulae are provided which can be used to update our age estimate as improved determinations for various quantities become available. Formulae are also provided which can be used to derive the age and its uncertainty for a globular cluster, given the absolute magnitude of the turn-off, or the point on the subgiant branch 0.05 mag redder than the turn-off.