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 subgian t stars from the HK survey of Beers et al. (1992) is given. Most of th ese stars have [Fe/H] less than or equal to -2.5. These photometric da ta have been taken and reduced using the same techniques as in our pre vious two uvby-beta catalogues. An error analysis has been made; typic al 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, respec tively. Our photometric data are also compared to the UBV photometry a nd spectral indices from the HK survey; satisfactory agreement is foun d indicating good quality for the two data sets. The stars are classif ied 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 met al-poor supergiants, three (possibly four) horizontal branch stars, on e blue subluminous star and two stars with ambiguous photometry have b een identified. Stellar parameters, such as E(b - y), M(V), delta M(V) , and T-eff, plus error estimates for these parameters, are provided u sing our photometric calibrations. Although all stars have galactic la titudes \b\ greater than or equal to 30 degrees many of them are signi ficantly 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 +/- 0.00 5 in contrast to the low value of E(B - V) similar or equal to 0.01 de rived from the Burstein & Heiles (1982) maps. The stars are analyzed i n the (b - y)(0), [Fe/H] diagram, and individual ages estimated using the isochrones of VandenBerg & Bell (1985) and of Bergbusch & VandenBe rg (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, whic h are perhaps analogous to the ''blue metal-poor'' stars of Preston et al. (1994), have been identified.