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 m ost 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-of f 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, lead ing 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] + 0.9 8], indicating that M68 is a 'young' globular cluster. A homogeneous s et of globular cluster age estimates with this precision would provide unprecedented insight into the formation of the Galactic halo.