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 open clusters , namely NGC 188 , NGC 6791 , Collinder 261 , Melotte 66 , Berkeley39 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 1998 ) .
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 ^ { +1 } _ { -2 } 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 uncertainties still affecting stellar models in the mass range 1.0 to 1.5 M _ { \odot } .
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 about 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 .