An accurate relative age estimator for globular clusters

Abstract

Globular cluster age estimates based on the absolute magnitude of the mainsequence 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.