Is the Galactic disc older than the halo?

The aim of this study is to infer the age of the Galactic disc by means of the ages of old open clusters, and comment on some recent claims that the Galactic disc may be older than the halo. To this purpose, we analyse the colour-magnitude diagrams (CMDs) of six very old open 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. 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 great as 12(-2)(+1) 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 to the fact that the above ages should be taken as provisional estimates, because of the many uncertainties still affecting stellar models in the mass range 1.0-1.5 M-circle dot. Despite this drawback of extant theory of stellar structure, if NGC 6791 and Berkeley 17 set a limit on the age of the Galactic disc, this component of the Milky Way can be as old as about 9-10 Gyr, but must surely be 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 disc, while other indicators - like white dwarfs - are perhaps more suited to this task.