Detailed Abundances of Two Very Metal-Poor Stars in Dwarf Galaxies
  The most metal-poor stars in dwarf spheroidal galaxies (dSphs) can show the
nucleosynthetic patterns of one or a few supernovae. These supernovae could
have zero metallicity, making metal-poor dSph stars the closest surviving links
to Population III stars. Metal-poor dSph stars also help to reveal the
formation mechanism of the Milky Way halo. We present the detailed abundances
from Keck/HIRES spectroscopy for two very metal-poor stars in two Milky Way
dSphs. One star, in the Sculptor dSph, has [Fe I/H] = -2.40. The other star, in
the Ursa Minor dSph, has [Fe I/H] = -3.16. Both stars fall in the previously
discovered low-metallicity, high-[alpha/Fe] plateau. Most abundance ratios of
very metal-poor stars in these two dSphs are largely consistent with very
metal-poor halo stars. However, the abundances of Na and some r-process
elements lie at the lower end of the envelope defined by inner halo stars of
similar metallicity. We propose that the metallicity dependence of supernova
yields is the cause. The earliest supernovae in low-mass dSphs have less gas to
pollute than the earliest supernovae in massive halo progenitors. As a result,
dSph stars at -3 < [Fe/H] < -2 sample supernovae with [Fe/H] << -3, whereas
halo stars in the same metallicity range sample supernovae with [Fe/H] ~ -3.
Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher
metallicity in dSphs than in the progenitors of the inner halo.
