The Ages of 55 Globular Clusters as Determined Using an Improved delta
  V_TO^HB Method Along with Color-Magnitude Diagram Constraints, and Their
  Implications for Broader Issues
  Ages have been derived for 55 globular clusters (GCs) from overlays of
isochrones onto the turnoff photometry, assuming distances based on fits of
zero-age horizontal branch (ZAHB) models to the lower bound of the observed
distributions of HB stars. The error bar arising just from the "fitting" of
ZAHBs and isochrones is ~ +/- 0.25 Gyr, while that associated with distance and
chemical abundance uncertainties is ~ +/- 1.5-2 Gyr. Ages vary from mean values
of ~12.5 Gyr at [Fe/H] < -1.7 to ~11 Gyr at [Fe/H] > -1.0. At intermediate
metallicities, the age-metallicity relation (AMR) appears to be bifurcated: one
branch apparently contains clusters with disk-like kinematics, whereas the
other branch is populated by clusters with halo-type orbits. There is no
apparent dependence of age on Galactocentric distance (R_G) nor is there a
clear correlation of HB type with age. Subtle variations in the subgiant branch
(SGB) slopes of [Fe/H] < -1.5 GCs are tentatively attributed to helium
abundance differences. Curiously, GCs with steep "M13-like" SGBs tend to be
massive systems, located at small R_G, that show the strongest evidence for
multiple stellar populations. The others are typically low-mass systems that,
at the present time, should not be able to retain the matter lost by
mass-losing stars. The apparent separation of the two groups in terms of their
present-day gas retention properties is difficult to understand if all GCs were
initial ~20 times their current masses. The lowest mass systems may have never
been able to retain enough gas to produce a significant population of
second-generation stars; in this case, the observed light element abundance
variations were presumably present in the gas out of which the observed cluster
stars formed.
