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.
(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 +/- 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 +/-
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 +/- 0.02, midway between the range of
values empirically derived from smaller and/or less homogeneous 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 +/- 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 +/- 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 +/- 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.
