Detailed Abundances of Two Very Metal-poor Stars in Dwarf Galaxies

The most metal-poor stars in dwarf spheroidal galaxies (dSphs) can show the nucleosynthetic patterns of one or a few supernovae. These supernovae could have zero metallicity, making metal-poor dSph stars the closest surviving links to Population III stars. Metal-poor dSph stars also help to reveal the formation mechanism of the Milky Way halo. We present the detailed abundances from Keck/HIRES spectroscopy for two very metal-poor stars in two Milky Way dSphs. One star, in the Sculptor dSph, has [Fe I/H] = −2.40. The other star, in the Ursa Minor dSph, has [Fe I/H] = −3.16. Both stars fall in the previously discovered low-metallicity, high-[α/Fe] plateau. Most abundance ratios of very metal-poor stars in these two dSphs are largely consistent with very metal-poor halo stars. However, the abundances of Na and some r-process elements lie at the lower end of the envelope defined by inner halo stars of similar metallicity. We propose that the metallicity dependence of supernova yields is the cause. The earliest supernovae in low-mass dSphs have less gas to pollute than the earliest supernovae in massive halo progenitors. As a result, dSph stars at −3 < [Fe/H] < −2 sample supernovae with [Fe/H] ≪ −3, whereas halo stars in the same metallicity range sample supernovae with [Fe/H] ∼ −3. Consequently, enhancements in [Na/Fe] and [r/Fe] were deferred to higher metallicity in dSphs than in the progenitors of the inner halo. Subject headings: galaxies: dwarf — galaxies: abundances — galaxies: evolution — Local Group