The solar neighbourhood age-metallicity relation - Does it exist?

We derive stellar ages, from evolutionary tracks, and metallicities, from S tromgren photometry, for a sample of 5828 dwarf and sub-dwarf stars from th e Hipparcos Catalogue. This stellar disk sample is used to investigate the age-metallicity diagram in the solar neighbourhood. Such diagrams are often used to derive a so called age-metallicity relation. Because of the size o f our sample, we are able to quantify the impact on such diagrams, and deri ved relations, due to different selection effects. Some of these effects ar e of a more subtle sort, giving rise to erroneous conclusions. In particula r we show that [1] the age-metallicity diagram is well populated at all age s and especially that old, metal-rich stars do exist, [2] the scatter in me tallicity at any given age is larger than the observational errors, [3] the exclusion of cooler dwarf stars from an age-metallicity sample preferentia lly excludes old, metal-rich stars, depleting the upper right-hand corner o f the age-metallicity diagram, [4] the distance dependence found in the Edv ardsson et al. sample by Garnett & Kobulnicky is an expected artifact due t o the construction of the original sample. We conclude that, although some of it can be attributed to stellar migration in the galactic disk, a large part of the observed scatter is intrinsic to the formation processes of sta rs.