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

We derive stellar ages, from evolutionary tracks, and metallicities,from Strömgren photometry, for a sample of 5828 dwarf and sub-dwarfstars from the Hipparcos Catalogue. This stellar disk sample is used toinvestigate the age-metallicity diagram in the solar neighbourhood. Suchdiagrams are often used to derive a so called age-metallicity relation.Because of the size of our sample, we are able to quantify the impact onsuch diagrams, and derived relations, due to different selectioneffects. Some of these effects are of a more subtle sort, giving rise toerroneous conclusions. In particular we show that [1] theage-metallicity diagram is well populated at all ages and especiallythat old, metal-rich stars do exist, [2] the scatter in metallicity atany given age is larger than the observational errors, [3] the exclusionof cooler dwarf stars from an age-metallicity sample preferentiallyexcludes old, metal-rich stars, depleting the upper right-hand corner ofthe age-metallicity diagram, [4] the distance dependence found in theEdvardsson et al. sample by Garnett & Kobulnicky is an expectedartifact due to the construction of the original sample. We concludethat, although some of it can be attributed to stellar migration in thegalactic disk, a large part of the observed scatter is intrinsic to theformation processes of stars. Based on results from the ESA Hipparcossatellite. Full Table 1 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/377/911