The Age of the Oldest Globular Clusters

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Abstract
The age of three of the oldest clusters—M15, M68, M92—has been redetermined. We use the latest equation of state (EOS) and opacity data available for calculating both isochrones and zero-age horizontal branches and employ the brightness difference between turnoff and horizontal branch to determine the cluster age. Our procedure and the uncertainties in obtaining absolute ages are discussed in detail. We find M68 to be 12.2 ± 1.8 Gyr old; M15 appears to be either slightly younger or 2 Gyr older, depending on whether its age is determined differentially or directly. We discuss this discrepancy and argue that the lower age is within the real observational uncertainties. M92 is found to be slightly younger than M15 by about 0.5 Gyr. These ages are smaller by 1-2 Gyr compared to earlier work, and even smaller ages are possible. Our results help to reconcile cluster ages with recent results on the age of the universe determined from the Hubble constant.
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