THE HORIZONTAL-BRANCH STARS IN GLOBULAR-CLUSTERS .2. THE 2ND-PARAMETER PHENOMENON

Using synthetic horizontal-branch models, we have investigated the ori gin of the systematic variation in horizontal-branch (HB) morphology w ith galactocentric distance (R(G)) among globular clusters. The variat ions in He abundance, CNO abundance, and core mass required separately to explain this effect are inconsistent with either the observed prop erties of the RR Lyrae variables or the observed main-sequence turnoff s in the clusters. There is also no clear evidence that the trend with R(G) is related to the central concentrations, central densities, or absolute magnitudes of the clusters. The variations in cluster age req uired to explain this effect are not in conflict with any observations . A detailed comparison of our synthetic HB calculations with pairs of clusters of very different HB morphology but similar [Fe/H] reveals r easonably good agreement between the age differences inferred from HB morphology and the main-sequence turnoff. The major source of uncertai nty is the need for ad hoc hypotheses in the modeling of the HB morpho logies of a few peculiar clusters (e.g., NGC 6752). Nonetheless, there is firm evidence for age variations of several gigayears (as much as approximately 5 Gyr) among the halo globular clusters. Our results sup port the hypothesis of Searle & Zinn that the inner halo is more unifo rm in age and is older in the mean than the outer halo, and we estimat e this difference to be approximately 2 Gyr.