An accurate relative age estimator for globular clusters

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