1998A&A...333..505B

A new procedure is described to derive homogeneous relative ages from the Color-Magnitude Diagrams (CMDs) of Galactic globular clusters (GGCs). It is based on the use of a new observable, {DELTA}V^0.05 , namely the difference in magnitude between an arbitrary point on the upper main sequence (V_+0.05 -the V magnitude of the MS-ridge, 0.05mag redder than the Main Sequence (MS) Turn-off, (TO)) and the horizontal branch (HB). The observational error associated to {DELTA}V^0.05 is substantially smaller than that of previous age-indicators, keeping the property of being strictly independent of distance and reddening and of being based on theoretical luminosities rather than on still uncertain theoretical temperatures. As an additional bonus, the theoretical models show that {DELTA}V^0.05 has a low dependence on metallicity. Moreover, the estimates of the relative age so obtained are also sufficiently invariant (to within ~±1Gyr) with varying adopted models and transformations. Since the difference in the color difference {DELTA}(B-V)_TO,RGB (VandenBerg, Bolte and Stetson 1990 -VBS, Sarajedini and Demarque 1990 -SD) remains the most reliable technique to estimate relative cluster ages for clusters where the horizontal part of the HB is not adequately populated, we have used the differential ages obtained via the "vertical" {DELTA}V^0.05 parameter for a selected sample of clusters (with high quality CMDs, well populated HBs, trustworthy calibrations) to perform an empirical calibration of the "horizontal" observable in terms of [Fe/H] and age. A direct comparison with the corresponding calibration derived from the theoretical models reveals the existence of clear-cut discrepancies, which call into question the model scaling with metallicity in the observational planes. Starting from the global sample of considered clusters, we have thus evaluated, within a homogeneous procedure, relative ages for 33 GGCs having different metallicity, HB-morphologies, and galactocentric distances. These new estimates have also been compared with previous latest determinations (Chaboyer, Demarque and Sarajedini (1996ApJ...459..558C), and Richer et al. (1996ApJ...463..602R)). The distribution of the cluster ages with varying metallicity and galactocentric distance are briefly discussed: (a) there is no direct indication for any evident age-metallicity relationship; (b) there is some spread in age (still partially compatible with the errors), and the largest dispersion is found for intermediate metal-poor clusters; (c) older clusters populate both the inner and the outer regions of the Milky Way, while the younger globulars are present only in the outer regions, but the sample is far too poor to yield conclusive evidences.