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) for which Hubble Space Telescope Advanced Camera for Surveys photometry is publicly available. For most of them, the assumed distances are based on fits of theoretical zero-age horizontal-branch (ZAHB) loci to the lower bound of the observed distributions of HB stars, assuming reddenings from empirical dust maps and metallicities from the latest spectroscopic analyses. The age of the isochrone that provides the best fit to the stars in the vicinity of the turnoff (TO) is taken to be the best estimate of the cluster age. The morphology of isochrones between the TO and the beginning part of the subgiant branch (SGB) is shown to be nearly independent of age and chemical abundances. For well-defined color-magnitude diagrams (CMDs), 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. The oldest GCs in our sample are predicted to have ages of ≈13.0 Gyr (subject to the aforementioned uncertainties). However, the main focus of this investigation is on relative GC ages. In conflict with recent findings based on the relative main-sequence fitting method, which have been studied in some detail and reconciled with our results, ages are found to vary from mean values of ≈12.5 Gyr at [Fe/H] −1.7 to ≈11 Gyr at [Fe/H] −1. 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, which is displaced to lower [Fe/H] values by ≈0.6 dex at a fixed age, is populated by clusters with halo-type orbits. The dispersion in age about each component of the AMR is ∼±0.5 Gyr. There is no apparent dependence of age on Galactocentric distance (R G ) nor is there a clear correlation of HB type with age. As previously discovered in the case of M3 and M13, subtle variations have been found in the slope of the SGB in the CMDs of other metal-poor ([Fe/H] −1.5) GCs. They have been tentatively attributed to clusterto-cluster differences in the abundance of helium. Curiously, GCs that have relatively steep "M13-like" SGBs tend to be massive systems, located at small R G , that show the strongest evidence of in situ formation of multiple stellar populations. The clusters in the other group are typically low-mass systems (with 2-3 exceptions, including M3) that, at the present time, should not be able to retain the matter lost by mass-losing stars due either to the development of GC winds or to ram-pressure stripping by the halo interstellar medium. The apparent separation of the two groups in terms of their present-day gas retention properties is difficult to understand if all GCs were initially ∼20 times their current masses. The lowest-mass systems, in particular, may have never been massive enough to retain enough gas to produce a significant population of second-generation stars. In this case, the observed light element abundance variations, ...

Globular clusters of NGC 3115 in the near-infrared

“…The model comparison also suggests that the metal-rich GCs have slightly younger ages than the blue/metal-poor ones. Although this conclusion is subject to model uncertainties, the size of the age difference is consistent with that found between the metal-rich and metal-poor GCs in the Milky Way (VandenBerg et al 2013).…”

Evolution of long-lived globular cluster stars

“…Therefore, the number of nuclear active stars within the cluster evolves with time because of stellar evolution effects alone. We present our results for 9 and 13 Gyr old synthetic clusters, which covers the age spread derived for Galactic GCs (e.g., Marín-Franch et al 2009;VandenBerg et al 2013).…”

Evolution of long-lived globular cluster stars

“…This cluster is relatively metalpoor ([Fe/H] = −1.54, Campbell et al 2013). It is also relatively old, as estimates of its age vary between ∼12.5 ± 0.25 Gyr and 13.4 ± 1.1 Gyr (according to VandenBerg et al 2013 andGratton et al 2003 respectively). Therefore, its position in the age-Δ[Na/Fe] ini (AGB) plane (Fig.…”