A Precision Age Determination Technique for Globular Clusters

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Globular cluster age estimates based on the absolute magnitude of the main sequence turn-off () are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining in observed colour-magnitude diagrams leads to a large error in the derived age. In this paper, we advocate the use of the absolute magnitude of the point which is brighter than the turn-off and 0.05 mag redder () as a precision age indicator. It is easy to measure this point on observed colour-magnitude diagrams, leading to small observational error bars. Furthermore, an extensive Monte Carlo calculation indicates that the theoretical uncertainty in is similar to. As a result, ages derived using are at least a factor of 2 more precise than those derived using. This technique is applied to the globular cluster M68 and an age of 12.8± 0.3Gyr is derived (assuming = 0.20 + 0.98), indicating that M68 is a `young' globular cluster. A homogeneous set of globular cluster age estimates with this precision would provide unprecedented insight into the formation of the Galactic halo.