Technique for Globular Clusters

Globular cluster age estimates based on the absolute magnitude of the main sequence turn-off (Mv(TO)) are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining M v(TO) in observed colourmagnitude 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±0.3Gyr 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.