Metallicity and absolute magnitude calibrations for UBV photometry

Calibrations are presented here for metallicity ([Fe/H]) in terms of the ultraviolet excess, [delta(U - B) at B - V = 0.6, hereafter delta(0.6)], and also for the absolute visual magnitude (M-v) and its difference with respect to the Hyades (Delta M-V(H)) in terms of delta(0.6) and (B - V), making use of high-resolution spectroscopic abundances from the literature and Hipparcos parallaxes. The relation [Fe/H]-delta(0.6) has been derived for dwarf plus turn-off stars, and also for dwarf, turn-off, plus subgiant stars classified using the M-V-(B - V)(0) plane of Fig. 11, which is calibrated with isochrones from Bergbusch & VandenBerg (and also VandenBerg & Clem). The [Fe/H]-delta(0.6) relations 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-beta photometry and spectroscopic [Fe/H](spec) values 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-V calibration is presented, based on Hipparcos parallaxes with sigma(pi)/pi <= 0.1 and with a dispersion of +/- 0.24 in M-v. This hybrid M-v calibration contains delta(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 <= delta(0.6) <= +0.29 and 3.44 <= M-v <= 7.23. For dwarf and turn-off stars, the relation for Delta M-V(H) is revised and updated in terms of (B - V) and delta(0.6), for the ranges of -0.10 <= delta(0.6) <= +0.29, and +0.49 <= (B - V)(0) <= +0.89, again making use of Hipparcos parallaxes with sigma(pi)/pi <= 0.1. These parallaxes for metal-poor dwarf and turn-off stars in our sample reveal that the difference of Delta M-V(H)(B - V) relative to Hyades at (B-V) = +0.70 should be 1.37 mag, instead of the 1.58 mag given by Laird et al. In general, Hipparcos parallaxes are larger than ground-based ones, causing a divergence of our Delta M-V(H)(B - V, delta(0.6)) V relation (the solid line in Fig. 15b), from the one of Laird et al. (the dashed line) for the range +0.10 <= delta(0.6) <= +0.29; our absolute magnitudes are fainter, as has been confirmed for local subdwarfs by Reid. Our final calibrations for Delta M-V(H)(B - V, delta(0.6)), equations (16) and (17), are third-order polynomials in delta(0.6), pass through the origin, and provide photometric distances in reasonable agreement with those obtained directly from Hipparcos parallaxes (Fig. 18).