uvby-beta photometry of high-velocity and metal-poor stars .8. Stars of very low metal abundance

A catalogue of uvby-beta photometry for 87 of the turn-off and subgiant stars from the HK survey of Beers et al. (1992) is given. Most of these stars have [Fe/H] less than or equal to -2.5. These photometric data have been taken and reduced using the same techniques as in our previous two uvby-beta catalogues. An error analysis has been made; typical mean observational errors for a star with V = 14.(m)2 are 0.008, 0.007, 0.011, 0.010 and 0.012 in V, (b - y), m(1), c(1) and beta, respectively. Our photometric data are also compared to the UBV photometry and spectral indices from the HK survey; satisfactory agreement is found indicating good quality for the two data sets. The stars are classified in reddening-free diagrams of the uvby-beta system; several of the stars are found to have been mis-classified in the HK survey. Two metal-poor supergiants, three (possibly four) horizontal branch stars, one blue subluminous star and two stars with ambiguous photometry have been identified. Stellar parameters, such as E(b - y), M(V), delta M(V), and T-eff, plus error estimates for these parameters, are provided using our photometric calibrations. Although all stars have galactic latitudes \b\ greater than or equal to 30 degrees many of them are significantly reddened with E(b - y) ranging from 0.02 to 0.15. The average reddening towards the South Galactic Pole region, b < -60 degrees, is E(b - y) = 0.027 +/- 0.004 corresponding to E(B - V) = 0.036 +/- 0.005 in contrast to the low value of E(B - V) similar or equal to 0.01 derived from the Burstein & Heiles (1982) maps. The stars are analyzed in the (b - y)(0), [Fe/H] diagram, and individual ages estimated using the isochrones of VandenBerg & Bell (1985) and of Bergbusch & VandenBerg (1992). These very metal-poor stars are found to be coeval, within 1-2 Gyr, with the halo and thick-disk stars studied previously, with a mean age in excess of 18 Gyr. Several blue ''thick-disk'' stars, which are perhaps analogous to the ''blue metal-poor'' stars of Preston et al. (1994), have been identified.