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- Hβ 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. σW= 32 ± 5 km s−1, 〈Vrot〉= 154 ± 6 km s−1 and 〈[M/H]〉=−0.55 ± 0.03 dex for these thick-disc stars, which is in agreement with values from previous studies of the thick disc. α-element abundances which are available for some of these thick-disc stars show the typical α-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, Δ([M/Fe])/Δ(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]≳-0.2 dex. However, two of these stars have [M/H]~-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 ≲ age ≲ 13 Gyr.