Abundance patterns in the Draco, Sextans, and Ursa Minor dwarf spheroidal galaxies

The Keck I telescope and the High Resolution Echelle Spectrometer (HIRES) h ave been used to obtain spectra for red giant stars belonging to the Draco, Sextans, and Ursa Minor dwarf spheroidal (dSph) galaxies. An analysis of t hese spectra is presented, along with abundance ratios for more than 20 ele ments. The resulting database of element abundances for 17 dSph stars is th e most extensive yet assembled for stars in such environments. Our principa l findings are summarized as follows. (1) There is unambiguous evidence for a large internal dispersion in metallicity in all three galaxies : our pro gram stars span a range of Delta [Fe/H] = 1.53, 1.40, and 0.73 dex in Draco , Sextans, and Ursa Minor, respectively. (2) The abundance patterns among t he dSph stars are remarkably uniform, suggesting that three galaxies have s imilar nucleosynthetic histories and, presumably, similar initial mass func tions. (3) A comparison of the measured element abundance ratios for our sa mple of dSph stars with published values for Galactic halo and disk field s tars suggests that the dSph galaxies have dex, 0.02 less than or similar to [alpha /Fe] less than or similar to 0.13 whereas the halo field star sampl e has [alpha /Fe] similar to 0.28 dex over the same range in metallicity. ( 4) The most metal-rich dSph stars in our sample have [Y/Fe] abundances that are significantly lower than those measured for halo field stars of simila r metallicity, while the measured [Ba/Eu] ratios for the dSph stars suggest that the early chemical evolution of these galaxies was dominated by the r -process. Taken together, these results suggest that the Galactic halo is u nlikely to have assembled, in its entirety, through the disruption of dwarf galaxies similar to the low-luminosity, [LV] = 3 x 10(5) L-V,L-., dSph gal axies studied here. We also note that the measured [Zn/Fe] abundance ratios for the dSph stars are lower than those of damped Ly alpha systems having low levels of dust depletion by roughly an order of magnitude. The first high-resolution abundance analysis for the distant Galactic globu lar cluster NGC 2419 is also presented. From a HIRES spectrum of a single r ed giant, we find a metallicity of [Fe/H] = -2.32 +/- 0.11 dex. This is sli ghtly lower than, but still consistent with, published estimates based on l ow-resolution spectroscopy. With the possible exception of a slight enhance ment in the abundances of some heavy elements such as Ce, Nd, Y, and Ba, th e observed abundance pattern closely resembles those exhibited by red giant s in M92: a nearby, well-studied globular cluster of nearly identical metal licity.