  The entire evolution of the Milky Way, including its mass-assembly and
star-formation history, is imprinted onto the chemo-dynamical distribution
function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase
space spanned by position, velocity, and elemental abundance ratios. In
particular, the chemo-dynamical distribution functions for low-mass stars
(e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the
Milky Way's formation, since their main-sequence lifetimes approach or exceed
the age of the universe. A basic tenet of essentially all previous analyses is
that the stellar metallicity, usually parametrized as [Fe/H], is conserved over
time for main-sequence stars (at least those that have not been polluted due to
mass transfer from binary companions). If this holds true, any correlations
between metallicity and kinematics for long-lived main-sequence stars of
different masses, effective temperatures, or spectral types must strictly be
the same, since they reflect the same mass-assembly and star-formation
histories. By analyzing a sample of nearby metal-poor halo and thick-disk stars
on the main sequence, taken from Data Release 8 of the Sloan Digital Sky
Survey, we find that the median metallicity of G-type dwarfs is systematically
higher (by about 0.2 dex) than that of K-type dwarfs having the same median
rotational velocity about the Galactic center. If it can be confirmed, this
finding may invalidate the long-accepted assumption that the atmospheric
metallicities of long-lived stars are conserved over time.
