A Differential Abundance Analysis of Very Metal-Poor Stars
  We have performed a differential, line-by-line, chemical abundance analysis,
ultimately relative to the Sun, of nine very metal-poor main sequence halo
stars, near [Fe/H]=$-$2 dex. Our abundances range from
$-2.66\leq\mathrm{[Fe/H]}\leq-1.40$ dex with conservative uncertainties of 0.07
dex. We find an average [$\alpha$/Fe]$=0.34\pm0.09$ dex, typical of the Milky
Way. While our spectroscopic atmosphere parameters provide good agreement with
HST parallaxes, there is significant disagreement with temperature and gravity
parameters indicated by observed colors and theoretical isochrones. Although a
systematic underestimate of the stellar temperature by a few hundred degrees
could explain this difference, it is not supported by current effective
temperature studies and would create large uncertainties in the abundance
determinations. Both 1D and $\langle$3D$\rangle$ hydrodynamical models combined
with separate 1D non-LTE effects do not yet account for the atmospheres of real
metal-poor MS stars, but a fully 3D non-LTE treatment may be able to explain
the ionization imbalance found in this work.
