On the galactic disc age-metallicity relation

A comparison is made between the age-metallicity relation obtained from four different types of studies: F and G stars in the Solar Neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique, and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be -0.09 dex/kpc and to be most likely constant in time. We do not correct for the vertical gradient, since its existence and value are not firmly established. Stars and clusters trace a similar age-metallicity relation, showing an excess of rather metal-rich objects in the age range 5-9 Gyr. Galactic structure studies tend to give a more metal-poor relation than chemical evolution models. Both relations do not explain the presence of old, relatively metal-rich stars and clusters. This might be due to uncertainties in the ages of the local stars, or to pre-enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.