ON THE RELATIVE AGES OF GALACTIC GLOBULAR-CLUSTERS - A NEW OBSERVABLE, A SEMIEMPIRICAL CALIBRATION AND PROBLEMS WITH THE THEORETICAL ISOCHRONES

A new procedure is described to derive homogeneous relative ages from the Color-Magnitude Diagrams (CMDs) of Galactic globular clusters (GGC s). It is based on the use of a new observable, Delta V-0.05, namely t he difference in magnitude between an arbitrary point on the upper mai n sequence (V+0.05 -the V magnitude of the MS-ridge, 0.05 mag redder t han the Main Sequence (MS) Turn-off, (TO)) and the horizontal branch ( HB). The observational error associated to Delta V-0.05 is substantial ly smaller than that of previous age-indicators, keeping the property of being strictly independent of distance nd reddening and of being ba sed on theoretical luminosities rather than on still uncertain theoret ical temperatures. As an additional bonus, the theoretical models show that Delta V-0.05 has low dependence on metallicity. Moreover, the es timates of the relative age so obtained are also sufficiently invarian t (to within similar to +/-1Gyr) with varying adopted models and trans formations. Since the difference in the color difference Delta(B - V)( TO,RGB) (VandenBerg, BoIte and Stetson 1990-VBS, Sarajedini and Demarq ue 1990-SD) remains the most reliable technique to estimate relative c luster ages for clusters where the horizontal part of the HE is not ad equately populated, we have used the differential ages obtained via th e ''vertical'' Delta V-0.05 parameter for a selected sample of cluster s (with high quality CMDs, well populated HBs, trustworthy calibration s) to perform an empirical calibration of the ''horizontal'' observabl e in terms of [Fe/H] and age. A direct comparison with the correspondi ng calibration derived from the theoretical models reveals the existen ce of clear-cut discrepancies, which call into question the model scal ing with metallicity in the observational planes. Starting from the gl obal sample of considered clusters, we have thus evaluated, within a h omogeneous procedure, relative ages for 33 GGCs having different metal licity, HB-morphologies, and galactocentric distances. These new estim ates have also been compared with previous latest determinations (Chab oyer, Demarque and Sarajedini 1996, and Richer et al. 1996). The distr ibution of the cluster ages with varying metallicity and galactocentri c distance are briefly discussed: (a) there is no direct indication fo r any evident age-metallicity relationship; (b) there is some spread i n age (still partially compatible with the errors), and the largest di spersion is found for intermediate metal-poor clusters; (c) older clus ters populate both the inner and the outer regions of the Milky Way, w hile the younger globulars are present only in the outer regions, but the sample is far too poor to yield conclusive evidences.