Aim of this study is to infer the age of the Galactic Disk by means of the
ages of old open clusters, and comment on some recent claims that the Galactic
Disk can be older than the Halo. To this purpose, we analyze the
Color-Magnitude Diagrams (CMDs) of six very old clusters, namely NGC 188, NGC
6791, Collinder 261, Melotte 66, Berkeley 39 and Berkeley 17, and determine
their ages. For each cluster we use the most recent photometric and
spectroscopic data and metallicity estimates. The ages are derived from the
isochrone fitting method using the stellar models of the Padua library
(Bertelli et al . 1994, Girardi et al. 1999). We find that the ages of these
clusters fall in the range 4 to 9-10 Gyr: Melotte 66 is the youngest whereas
NGC 6791 and Berkeley 17 have ages of about 9-10 Gyr. Previous estimates for
Berkeley 17 indicated an age as old as 12 Gyr, almost falling within the range
of classical globular clusters. In our analysis, this cluster is always very
old but perhaps somewhat younger than in previous studies. However we call
attention on the fact that the above ages are to be taken as provisional
estimates, because of the many uncertaintes still affecting stellar models in
the mass range 1.0 to 1.5 Mo. Despite this drawback of extant theory of stellar
structure, if NGC 6791 and Berkeley 17 set the limit to the age of the Galactic
Disk, this component of the Milky Way can be as old as 9-10 Gyr, but surely
younger than the Galactic Halo, at least as inferred from recent determinations
of the age of globular clusters. Finally, it is worth recalling that open
clusters can only provide a lower limit to the age of the Galactic Disk, while
other indicators - like White Dwarfs - are perhaps more suited to this task.Comment: 16 pages, 11 figure, accepted for publication in MNRA
