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 . ( B - V ) - temperature relations are derived from 1746 stars between [ Fe/H ] = -0.5 and +0.6 and 372 stars within 0.05 dex of solar abundance ; for T _ { e } = 5770 K , the solar color is B - V = 0.652 \pm 0.002 ( s.e.m . ) .
From over 500 cool dwarfs between [ Fe/H ] = -0.5 and +0.5 , \Delta ( B - V ) / \Delta [ Fe/H ] at fixed M _ { V } = 0.213 \pm 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 M _ { V } / \Delta [ Fe/H ] at fixed B - V = 0.98 \pm 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 ( [ Fe/H ] = +0.13 ) with no reddening has ( m - M ) _ { 0 } = 3.33 \pm 0.02 and M67 , with E ( B - V ) = 0.041 , A _ { V } = 3.1E ( B - V ) , and [ Fe/H ] = 0.00 , has ( m - M ) _ { 0 } = 9.71 \pm 0.02 ( s.e.m ) , 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 \pm 0.025 and [ Fe/H ] = +0.39 \pm 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 \pm 0.09 , internal and systematic errors included .