Ages of globular clusters: a new approach

We have applied a new method to analyze the horizontal branch (HB) morphology in relation to the distribution of stars near the red giant branch (RGB) tip for the globular clusters M22, M5, M68, M107, M72, M92, M3 and 47 Tuc. This new method permits determination of the cluster ages to greater accuracy than conventional isochrone fitting. Using the method in conjunction with our new high-quality photometric data for RGB and HB stars in the first five of these clusters, we discuss the origins of the spread in color on the HB and its relation to the ‘second parameter’ problem. The oldest clusters in our sample are found to have relatively low ages (13.5 ± 2 Gyr). A 1σ uncertainty in each of the parameters of mass and helium content combined with the effects of helium diffusion gives a lower limit for the age of the oldest clusters of 9.7 Gyr.