UVBY-BETA-PHOTOMETRY OF HIGH-VELOCITY AND METAL-POOR STARS .7. AGES OF HALO AND THICK-DISK FIELD STARS

Ages have now been determined for the turn-off stars of our two uvby - beta catalogues. As in Paper III, polynomial fittings have been used to interpolate the isochrones of VandenBerg and Bell (1985) in the c(0),(b - y)(0) plane, and as in Paper V, graphical interpolations for the isochrones of VandenBerg (1985) and of Bergbusch and VandenBerg (1992) in the delta M(V), log T-eff plane. The interstellar color excesses, E(b - y), and photometric metallicities, [Fe/H], of the stars have been determined using our previous photometric calibrations. Membership of the stars in the different stellar populations has been determined using the V-rot,[Fe/H] diagram, and Galactic orbits calculated for the halo stars employing the Galactic mass distribution of Alien and Santillan (1991). Combining with previous work and cleansing the samples of the more obvious binary-star contamination, ages remain for 71 hare stars and for 296 ''high-velocity disk'' stars of which 24 are from the nearly pure thick-disk sample of Paper VI. Our previous conclusions are mostly confirmed that there is good evidence for a cosmic age scatter of several Gyr within the halo field stars and that there is considerable overlap between the ages of the halo stars and the older disk stars. However, the previously found metallicity-age correlation for the halo field stars has mostly disappeared. In addition, evidence is found that the inner part of the halo is older and has a smaller age dispersion than the outer part, and that, on the average, the thick disk is only 1-2 Gyr younger than the halo having a mean age similar to that of the outer halo. These results are discussed and interpreted in terms of various formation scenarios for the thick disk, dual-halo models, and ''inside-out'' evolutionary schemes for the Galaxy proposed in the literature.