Ages of globular clusters: a new approach

A B ST R A C T W has applied a new m ethod to analyzethehorizontal branch (HB)m orphology in relation to distribution of starsnearthered giant branch (RGB) tipfortheglobular clusters to greater accuracy than conventionalisochrone �tting.

Abstract

We have used a new method to analyse 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 the determination of the cluster ages to greater accuracy than does 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 colour 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 1ϋ 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.