1996A&AS..117..317S

A catalogue of uvby-β 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]≤-2.5. These photometric data have been taken and reduced using the same techniques as in our previous two uvby-β catalogues. An error analysis has been made; typical mean observational errors for a star with V=14.2mag are 0.008, 0.007, 0.011, 0.010 and 0.012 in V, (b-y), m₁, c₁ and β, 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-β 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, δM_V, and T_ eff_, plus error estimates for these parameters, are provided using our photometric calibrations. Although all stars have galactic latitudes |b|≥30deg 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←60deg, 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)≃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 1-2Gyr, with the halo and thick-disk stars studied previously, with a mean age in excess of 18Gyr. Several blue "thick-disk" stars, which are perhaps analogous to the "blue metal-poor" stars of Preston et al. (1994), have been identified.