F stars: Evidence for ?two-dimensional? age-metallicity relation and a new light on the enrichment history of the solar neighbourhood

Abstract

From theuvby photometry and proper motions for about 5500 nearby F stars we have found the following: (i) F stars, taken in narrow ranges of metallicity, show at [Fe/H]<0 rather distinct cut-off in their distribution along the Main Sequence (MS) at the blue side, which is suggested to be an indication for the MS turn-off in stellar groups of fixed metallicity; (ii) the corresponding turn-off age from theoretical isochrones strongly correlates with the mean peculiar velocity of the turn-off stars; (iii) the sub-groups of stars of different colours have essentially the same mean peculiar velocity at low metallicity, but at high metallicity the velocities of the ‘red’ subgroups are much larger than those of the ‘blue’ ones. We argue that these properties of F stars lead to a ‘two-dimensional’ age-metallicity relation with the following main features: (i) a very large spread of metal abundance for old stars, (ii) narrowing of the metallicity range toward younger ages, (iii) increase of mean metallicity toward younger ages. This AMR seems to require a major revision of current models of the chemical evolution of the Galaxy: it suggests that the spatial distribution of metal abundance in the interstellar medium was initially highly inhomogeneous, the inhomogeneities being smoothed out and the mean metallicity being increased as the time went on. We also find an evidence for the evolution of the gaseous matter, from which the open clusters are formed, to be somehow decoupled from the evolution of the overall ISM.