The Chemical Evolution of the Ursa Minor Dwarf Spheroidal Galaxy

Abstract: 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.