2006MNRAS.371.1793K

Calibrations are presented here for metallicity ([Fe/H]) in terms of the ultraviolet excess, [δ(U - B) at B - V = 0.6, hereafter δ_0.6], and also for the absolute visual magnitude (M_V) and its difference with respect to the Hyades (ΔM^H_V) in terms of δ_0.6and (B - V), making use of high-resolution spectroscopic abundances from the literature and Hipparcos parallaxes. The relation [Fe/H]-δ_0.6has been derived for dwarf plus turn-off stars, and also for dwarf, turn-off, plus subgiant stars classified using the M_V-(B - V)₀plane of Fig. 11, which is calibrated with isochrones from Bergbusch & VandenBerg (and also VandenBerg & Clem). The [Fe/H]-δ_0.6relations in our equations (5) and (6) agree well with those of Carney, as can be seen from Fig. 5(a). Within the uncertainties, the zero-points, +0.13(±0.05) of equation (5) and +0.13(±0.04) of equation (6), are in good agreement with the photometric ones of Cameron and of Carney, and close to the spectroscopic ones of Cayrel et al. and of Boesgaard & Friel for the Hyades open cluster. Good quantitative agreement between our estimated [Fe/H] abundances with those from uvby-β photometry and spectroscopic [Fe/H]_specvalues demonstrates that our equation (6) can be used in deriving quality photometric metal abundances for field stars and clusters using UBV data from various photometric surveys.

For dwarf and turn-off stars, a new hybrid M_Vcalibration is presented, based on Hipparcos parallaxes with σ_π/π ≤ 0.1 and with a dispersion of ±0.24 in M_V. This hybrid M_Vcalibration contains δ_0.6 and (B - V) terms, plus higher order cross-terms of these, and is valid for the ranges of +0.37 ≤ (B - V)₀≤ +0.88,- 0.10 ≤ δ_0.6≤ +0.29 and 3.44 ≤ M_V≤ 7.23. For dwarf and turn-off stars, the relation for ΔM^H_V is revised and updated in terms of (B - V) and δ_0.6, for the ranges of -0.10 ≤ δ_0.6≤ +0.29, and +0.49 ≤ (B - V)₀≤ +0.89, again making use of Hipparcos parallaxes with σ_π/π ≤ 0.1. These parallaxes for metal-poor dwarf and turn-off stars in our sample reveal that the difference of ΔM^H_V_(B - V) relative to Hyades at (B - V) = +0.70 should be 1.37mag, instead of the 1.58mag given by Laird et al. In general, Hipparcos parallaxes are larger than ground-based ones, causing a divergence of our ΔM^H_V(B - V,δ_0.6) relation (the solid line in Fig. 15b), from the one of Laird et al. (the dashed line) for the range +0.10 ≤ δ_0.6≤ +0.29; our absolute magnitudes are fainter, as has been confirmed for local subdwarfs by Reid. Our final calibrations for ΔM^H_V(B - V, δ_0.6), equations (16) and (17)), are third-order polynomials in δ_0.6, pass through the origin, and provide photometric distances in reasonable agreement with those obtained directly from Hipparcos parallaxes (Fig. 18).