The Chemical Evolution of the Ursa Minor Dwarf Spheroidal Galaxy
  We present an abundance analysis based on high resolution spectra of 10 stars
selected to span the full range in metallicity in the Ursa Minor dwarf
spheroidal galaxy. We find [Fe/H] for the sample stars ranges from -1.35 to
-3.10 dex and establish the trends of the abundance ratios [X/Fe]. In key
cases, particularly for the alpha-elements, these resemble those for stars in
the outer part of the Galactic halo, especially at the lowest metallicities
probed. The n-capture elements show a r-process distribution over the full
range of Fe-metallicity. This suggests that the duration of star formation in
the UMi dSph was shorter than in other dSph galaxies. The derived ages for a
larger sample of UMi stars with more uncertain metallicities also suggest a
population dominated by uniformly old (~13 Gyr) stars, with a hint of an
age-metallicity relationship.
  In comparing our results for UMi, our earlier work in Draco, and published
studies of more metal-rich dSph Galactic satellites, there appears to be a
pattern of moving from a chemical inventory for dSph giants with [Fe/H] < -2
dex which is very similar to that of stars in the outer part of the Galactic
halo (enhanced alpha/Fe relative to the Sun, coupled with subsolar [X/Fe] for
the heavy neutron capture elements and r-process domination), switching to
subsolar alpha-elements and super-solar s-process dominated neutron capture
elements for the highest [Fe/H] dSph stars. The combination of low star
formation rates over a varying and sometimes extended duration that produced
the stellar populations in the local dSph galaxies with [Fe/H] > -1.5 dex leads
to a chemical inventory wildly discrepant from that of any component of the
Milky Way.
