Abundance Patterns in the Draco, Sextans, and Ursa Minor Dwarf Spheroidal Galaxies

Accepted for publication in the Astrophysical Journal The Keck I telescope and the High Resolution Echelle Spectrometer (HIRES) have been used to obtain spectra for red giant stars belonging to the Draco, Sextans and Ursa Minor dwarf spheroidal (dSph) galaxies. An analysis of these spectra is presented, along with abundance ratios for more than 20 elements. The resulting database of element abundances for 17 dSph stars is the most extensive yet assembled for stars in such environments. Our principal findings are summarized as follows: (1) There is unambiguous evidence for a large internal dispersion in metallicity in all three galaxies: our program stars span a range of ∆Fe/H = 1.53, 1.40 and 0.73 dex in Draco, Sextans and Ursa Minor, respectively. (2) The abundance patterns among the dSph stars are remarkably uniform, suggesting that three galaxies have similar nucleosynthetic histories and, presumably, similar initial mass functions. (3) A comparison of the measured element abundance ratios for our sample of dSph stars with published values for Galactic halo and disk field stars suggests that the dSph galaxies have 0.02 ∼ α/Fe ∼ 0.13 dex, whereas the halo field star sample has α/Fe ∼ 0.28 dex over the same range in metallicity. (4) The most metal-rich dSph stars in our sample have Y/Fe abundances which are significantly lower than those measured for halo