Abstract
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.
Highlights
The increasing number of detailed spectroscopic surveys investigating kinematic and chemical properties of dwarf spheroidal galaxies provide new opportunities to study their local formation environment in detail and the formation and evolution of galaxies in general
We presented a new data set of chemical abundances for the Ursa Minor (UMi) dwarf spheroidal (dSph), based on spectra taken with the Keck High Resolution Echelle Spectrograph (HIRES) spectrograph, as well as a new set of chemical evolution models for 12 elements
A data set combining our new measurements with data from previous studies is consistent with a scenario in which the very metal poor stars in the dSphs have αelement abundances generally comparable to those in the Galactic halo, in UMi, there is a gradual decrease in the α elements with increasing metallicity which indicates that the interstellar medium (ISM) was enriched by SN Ia
Summary
The increasing number of detailed spectroscopic surveys investigating kinematic and chemical properties of dwarf spheroidal (dSph) galaxies (see e.g., Tolstoy et al 2004; Helmi et al 2006; Koch et al 2006; Martin et al 2007; Walker et al 2009a) provide new opportunities to study their local formation environment in detail and the formation and evolution of galaxies in general Despite their low luminosities, their gravitational potentials had to be large enough to confine their gas for sufficient time to allow an extended period of star formation (SF) to occur.
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