Figure 4 shows that adopted theoretical scenario
with
is satisfactorily reproducing the CM diagram of both
test clusters with reasonable assumptions about the cluster
reddenings and distance moduli, i.e., within the currently accepted
uncertainties on these values (see, e.g., Reed et al. 1988;
Harris 1999; Sandquist et al. 1999).
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Figure 4: The best fit of HST CMD of M 30 (left Panel) and 47 Tuc (right panel) with the adopted theoretical isochrones |
One may notice that the best fits predict for M 30 and 47 Tuc an age of 14 and 12 Gyr, respectively. As we will discuss in the following, one cannot attribute too much weight to this absolute determination of the ages, because of systematic uncertainties. However, it appears of obvious relevance that a similar difference in ages, with 47 Tuc younger by a couple of Gyr than M 30, has been earlier suggested by Vandenberg et al. (1990) and supported by the recent investigation by Salaris & Weiss (1998).
As for absolute age determinations, the problem has been largely debated in the current literature and the situation is far from being firmly assessed. Several theoretical investigations have found that new "improved" physic inputs affect stellar models in the direction of decreasing the predicted ages (see for example Chaboyer & Kim 1995; VandenBerg et al. 1996; Cassisi et al. 1998, 1999). However, in the meantime evidences have been discussed raising serious doubts about the adequacy of such new input physics (Pols et al. 1998; Caputo et al. 1999; Castellani et al. 2000). Taking also into account that the possible (if not probable) efficiency of element sedimentation in globular cluster stars can further reduce the predicted ages (Chaboyer 1995; Castellani et al. 1997; Cassisi et al. 1998) one should regard the ages given by the adopted theoretical scenario as a reasonable upper limit for the cluster ages, real ages being possibly smaller by something of the order of 2 Gyr. However, one could be much more confident in the differential behaviour, which appears scarcely affected by the systematic uncertainties we are dealing with.
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