We present detailed chemical element abundance ratios of 17 elements
with eight ≤ Z ≤ 60 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/High-Resolution Echelle Spectrograph 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 [α/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 dwarf spheroidal, indicate that Ursa Minor 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.
