  We present detailed chemical element abundance ratios of 17 elements in three
metal poor stars in the Ursa Minor dwarf spheroidal galaxy, which we combine
with extant data from the literature to assess the predictions of a novel suite
of galaxy chemical evolution models. The spectroscopic data were obtained with
the Keck/HIRES instrument and revealed low metallicities of [Fe/H]=-2.12, -2.13
and -2.67 dex. While the most metal poor star in our sample shows an
overabundance of [Mn/Fe] and other Fe-peak elements, our overall findings are
in agreement with previous studies of this galaxy: elevated values of the
[alpha/Fe] ratios that are similar to, or only slightly lower than, the halo
values but with SN Ia enrichment at very low metallicity, as well as an
enhancement of the ratio of first to second peak neutron capture elements
[Y/Ba] with decreasing metallicity. The chemical evolution models which were
tailored to reproduce the metallicity distribution function of the dSph,
indicate that UMi had an extended star formation which lasted nearly 5 Gyr with
low efficiency and are able to explain the [Y/Ba] enhancement at low
metallicity for the first time. In particular, we show that the present day
lack of gas is probably due to continuous loss of gas from the system, which we
model as winds.
