Uvby-β photometry of high-velocity and metal-poor stars. VIII. Stars of very low metal abundance.

Details Uvby-β photometry of high-velocity and metal-poor stars. VIII. Stars of very low metal abundance. Uvby-β photometry of high-velocity and metal-poor stars. VIII. Stars of very low metal abundance.

Jun 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26as..117..317s&link_type=abstract

Astronomy and Astrophysics Supplement, v.117, p.317-334

Astronomy and Astrophysics

Astrophysics

33

Stars: Abundances, Stars: Distances, Stars: Fundamental Parameters, Dust, Extinction, Galaxy: Evolution, Galaxy: Halo

Scientific paper

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), m_1_, c_1_ 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.

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