Near IR photometry of the old open clusters Berkeley 17 and Berkeley 18. Probing the age of the galactic disc

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We report on near IR (J and K bands) observations of two 8 × 8 (arcmin)² regions centered on the old open clusters Berkeley 17 and Berkeley 18, for which only optical photometry (in B,V and I bands) exists. J and K photometry allows us to obtain an independent estimate of cluster metallicity by means of the relationship between the spectroscopic metallicity and the Red Giant Branch (RGB) slope calibrated by Tiede et al (1997). From the analysis of the colour magnitude diagram (CMD) and luminosity function (LF), Berkeley 17 turns out to have a metal content [Fe/H] -0.35. It is 9 Gyr old, suffers from a reddening E(B-V) = 0.58 mag and has an heliocentric distance of 2.5 kpc. Berkeley 17 comes out to be substantially younger than in previous work (age ≈ 13 Gyr). On the other hand Berkeley 18 is found to have solar metal abundance, and to be younger than Berkeley 17, with an age of about 4 Gyr. While we confirm Kaluzny (1997) reddening estimate, we significantly revise the distance of the cluster, which lies 4.5 kpc from the Sun. These results on two open clusters believed to be between the oldest put constraints on the age and the evolution of the Galactic Disc. The absence of clusters older than 8-9 Gyr suggests the possibility that the Galaxy underwent a star formation minimum between 13 and 10 Gyr ago.