An accurate relative age estimator for globular clusters

Globular cluster age estimates based on the absolute magnitude of the main-sequence turn-off [M_v(TO)] are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining M_v(TO) 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 [M_v(BTO)] 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 M_v(BTO) is similar to M_v(TO). As a result, ages derived using M_v(BTO) are at least a factor of 2 more precise than those derived using M_v(TO). This technique is applied to the globular cluster M68 and an age of 12.8 ± 0.3 Gyr is derived [assuming M_v(RR) = 0.20[Fe/H] + 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.