The Unevolved Main Sequence of Nearby Field Stars and the Open Cluster Distance Scale

The slope and zero point of the unevolved main sequence as a function of metallicity are investigated using a homogeneous catalog of nearby field stars with absolute magnitudes defined with revised Hipparcos parallaxes, Tycho- 2 photometry, and precise metallicities from high- dispersion spectroscopy. dB similar to V _- temperature relations are derived from 1746 stars between [Fc/H] = -0.5 and +0.6 and 372 stars within 0.05 dex of solar abundance; for T-eff = 5770 K, the solar color is B - V 0.652 +/- 0.002 (SEM). From over 500 cool dwarfs between [Fc/H] = -0.5 and +0.5, Delta(B - V)/Delta[Fe/H] at fixed M-V = 0.213 +/- 0.005, with a very weak dependence upon the adopted main-sequence slope with B - V at a given [Fe/H]. At Hyades metallicity, this translates into Delta wM(V)/Delta|Fe/H| at fixed B - V = 0.98 +/- 0.02, midway between the range of values empirically derived from smaller and/or less homogenous samples and model isochrones. From field stars of similar metallicity, the Hyades ([Fc/H] = +0.13) with no reddening has (m - M)(0) = 3.33 +/- 0.02 and M67, with E(B - V) = 0.041, A(V) = 3.1E(B - V), and [Fc/H] = 0.00, has (m - M)(0) = 9.71 +/- 0.02 (SEM), where the errors quoted refer to internal errors alone. At the extreme end of the age and metallicity scale, with E(B - V) = 0.125 +/- 0.025 and [Fe/H] = +0.39 +/- 0.06, comparison of the fiducial relation for NGC 6791 to 19 field stars with (B - V) above 0.90 and [Fe/H] = +0.25 or higher, adjusted to the metallicity of NGC 6791, leads to (m - M)(0) = 13.07 +/- 0.09, internal and systematic errors included.