THE AGES OF 55 GLOBULAR CLUSTERS AS DETERMINED USING AN IMPROVED $\Delta V^{\rm HB}_{\rm TO}$ METHOD ALONG WITH COLOR-MAGNITUDE DIAGRAM CONSTRAINTS, AND THEIR IMPLICATIONS FOR BROADER ISSUES

Abstract

Ages have been derived for 55 globular clusters (GCs) from overlays of\nisochrones onto the turnoff photometry, assuming distances based on fits of\nzero-age horizontal branch (ZAHB) models to the lower bound of the observed\ndistributions of HB stars. The error bar arising just from the "fitting" of\nZAHBs and isochrones is ~ +/- 0.25 Gyr, while that associated with distance and\nchemical abundance uncertainties is ~ +/- 1.5-2 Gyr. Ages vary from mean values\nof ~12.5 Gyr at [Fe/H] < -1.7 to ~11 Gyr at [Fe/H] > -1.0. At intermediate\nmetallicities, the age-metallicity relation (AMR) appears to be bifurcated: one\nbranch apparently contains clusters with disk-like kinematics, whereas the\nother branch is populated by clusters with halo-type orbits. There is no\napparent dependence of age on Galactocentric distance (R_G) nor is there a\nclear correlation of HB type with age. Subtle variations in the subgiant branch\n(SGB) slopes of [Fe/H] < -1.5 GCs are tentatively attributed to helium\nabundance differences. Curiously, GCs with steep "M13-like" SGBs tend to be\nmassive systems, located at small R_G, that show the strongest evidence for\nmultiple stellar populations. The others are typically low-mass systems that,\nat the present time, should not be able to retain the matter lost by\nmass-losing stars. The apparent separation of the two groups in terms of their\npresent-day gas retention properties is difficult to understand if all GCs were\ninitial ~20 times their current masses. The lowest mass systems may have never\nbeen able to retain enough gas to produce a significant population of\nsecond-generation stars; in this case, the observed light element abundance\nvariations were presumably present in the gas out of which the observed cluster\nstars formed.\n