Stellar chemical signatures and hierarchical galaxy formation

To compare the chemistries of stars in the Milky Way dwarf spheroidal (dSph) satellite galaxies with stars in the Galaxy, we have compiled a large sample of Galactic stellar abundances from the literature. When kinematic information is available, we have assigned the stars to standard Galactic components through Bayesian classification based on Gaussian velocity ellipsoids. As found in previous studies, the [alpha/Fe] ratios of most stars in the dSph galaxies are generally lower than similar metallicity Galactic stars in this extended sample. Our kinematically selected stars confirm this for the Galactic halo, thin-disk, and thick-disk components. There is marginal overlap in the low [alpha/Fe] ratios between dSph stars and Galactic halo stars on extreme retrograde orbits ( V The new and independent environments offered by the dSph galaxies also allow us to examine fundamental assumptions related to the nucleosynthesis of the elements. The metal-poor stars ([Fe/H] ≤ - 1.8) in the dSph galaxies appear to have lower [Ca/Fe] and [Ti/Fe] than [Mg/Fe] ratios, unlike similar metallicity stars in the Galaxy. Predictions from the α-process (α-rich freeze-out) would be consistent with this result if there have been a lack of hypernovae in dSph galaxies. The α-process could also be responsible for the very low Y abundances in the metal-poor stars in dSph's; since [La/Eu] ( and possibly [Ba/Eu]) are consistent with pure r- process results, the low [Y/Eu] suggests a separate r- process site for this light (first-peak) r- process element. We also discuss SNe II rates and yields as other alternatives, however. In stars with higher metallicities ([Fe/H] ≥ -1.8), contributions from the s-process are expected; [( Y, La, and Ba)/Eu] all rise as expected, and yet [Ba/Y] is still much higher in the dSph stars than similar metallicity Galactic stars. This result is consistent with s-process contributions from lower metallicity AGB stars in dSph galaxies, and is in good agreement with the slower chemical evolution expected in the low-mass dSph galaxies relative to the Galaxy, such that the build-up of metals occurs over much longer timescales. Future investigations of nucleosynthetic constraints ( as well as galaxy formation and evolution) will require an examination of many stars within individual dwarf galaxies. Finally, the Na-Ni trend reported in 1997 by Nissen & Schuster is confirmed in Galactic halo stars, but we discuss this in terms of the general nucleosynthesis of neutron-rich elements. We do not confirm that the Na-Ni trend is related to the accretion of dSph galaxies in the Galactic halo.