Boo-1137 - An Extremely Metal-Poor Star in the Ultra-Faint Dwarf
  Spheroidal Galaxy Bo\"{o}tes I
  We present high-resolution, high-S/N spectra of an extremely metal- poor
giant star Boo-1137 in the "ultra-faint" dwarf spheroidal galaxy (dSph) Bootes
I (absolute magnitude Mv ~ -6.3). With [Fe/H] = -3.7, this the most metal-poor
star yet identified in an ultra-faint dSph.
  Comparison of relative abundances, [X/Fe], for some 15 elements with those of
the extremely metal-poor giants of the Galactic halo shows Boo-1137 is "normal"
with respect to C and N, the odd-Z elements Na and Al, the Fe-peak elements,
and the n-capture elements Sr and Ba, in comparison with the bulk of the halo
with [Fe/H] < -3.0. The alpha- elements Mg, Si, Ca, and Ti are all higher by
Delta[X/Fe] ~ 0.2 than average halo values. Monte-Carlo analysis indicates
Delta[alpha/Fe] values this large are expected with probability ~ 0.02. The
abundance pattern in Boo-1137 suggests inhomogeneous chemical evolution,
consistent with the wide internal spread in Fe abundances we reported earlier.
The similarity of most of the Boo-1137 relative abundances with respect to halo
values, and the fact that the alpha-elements are all offset by a similar small
amount from the halo averages, points to the same underlying galaxy-scale
stellar initial mass function, but that Boo-1137 likely originated in a
star-forming region where the abundances reflect either poor mixing of
supernova (SN) ejecta, or poor sampling of the SN progenitor mass range, or
both.
