Kinematics, ages and metallicities for F- and G-type stars in the solar neighbourhood

A new metallicity distribution and an age-metallicity relation are presented for 437 nearby F and G turn-off and sub-giant stars selected from radial velocity data of Nidever et al. Photometric metallicities are derived from uvby- H beta photometry, and the stellar ages from the isochrones of Bergbusch & VandenBerg as transformed to uvby photometry using the methods of Clem et al. The X (stellar population) criterion of Schuster et al., which combines both kinematic and metallicity information, provides 22 thick-disc stars. sigma(W) = 32 +/- 5 km s(-1), < V-rot > = 154 +/- 6 km s(-1) and <[M/H]>=-0.55 +/- 0.03 dexfor these thick-disc stars, which is in agreement with values from previous studies of the thick disc. alpha-element abundances which are available for some of these thick-disc stars show the typical alpha-element signatures of the thick disc, supporting the classification procedure based on the X criterion. Both the scatter in metallicity at a given age and the presence of old, metal-rich stars in the age-metallicity relation make it difficult to decide whether or not an age-metallicity relation exists for the older thin-disc stars. For ages greater than 3 Gyr, our results agree with the other recent studies that there is almost no correlation between age and metallicity, Delta([M/Fe])/Delta(age) =-0.01 +/- 0.005 dex Gyr(-1). For the 22 thick-disc stars there is a range in ages of 7-8 Gyr, but again almost no correlation between age and metallicity. For the subset of main-sequence stars with extra-solar planets, the age-metallicity relation is very similar to that of the total sample, very flat, the main difference being that these stars are mostly metal-rich, [M/H]greater than or similar to-0.2 dex. However, two of these stars have [M/H]similar to-0.6 dex and have been classified as thick-disc stars. As for the total sample, the range in ages for these stars with extra-solar planetary systems is considerable with a nearly uniform distribution over 3 less than or similar to age less than or similar to 13 Gyr.