The chemical composition of halo stars on extreme orbits

Presented within is a fine spectroscopic analysis of 11 metal-poor (-2.15 < [Fe/H] < -1.00) dwarf stars on orbits that penetrate the outermost regions of the Galactic halo. Abundances for a select group of light metals (Na, Mg, Si, Ca, and Ti), Fe-peak nuclides (Cr, Fe, and Ni), and neutron-capture elements (Y and Ba) were calculated using line strengths measured from high-resolution (R approximate to 48,000), high signal-to-noise ratio (S/N approximate to 110 pixel(-1)) echelle spectra acquired with the Keck I 10 m telescope and HIRES spectrograph. Ten of the stars have apogalactica, a proxy for stellar birthplace, which stretch between 25 and 90 kpc; however, these "outer halo" stars exhibit strikingly uniform abundances. The average, Fe-normalized abundances-[[Mg/Fe]] = +0.23 +/- 0.09, [[Si/Fe]] = +0.24 +/- 0.10, [[Ca/Fe]] = +0.22 +/- 0.07, [[Ti/Fe]] = +0.20 +/- 0.08, [[Cr/Fe]] = 0.02 +/- 0.07, [[Ni/Fe]] = -0.09 +/- 0.07, and [[Ba/Fe]] = +0.01 +/- 0.12-exhibit little intrinsic scatter; moreover, the evolution of individual ratios (as a function of [Fe/H]) is generally consistent with the predictions of galactic chemical evolution models dominated by the ejecta of core-collapse supernovae. Only [[Y/Fe]] = -0.13 +/- 0.21 exhibits a dispersion larger than observational uncertainties, which suggests a different nucleosynthesis site for this element. It has been conjectured that stars on high-energy orbits-either those that penetrate the remote halo or ones with extreme retrograde velocities-were once associated with a cannibalized satellite galaxy. Such stars, as shown here, are indistinguishable from metal-poor dwarfs of the inner Galactic halo. The uniformity of the abundances, regardless of kinematic properties, suggests that physically, spatially, and temporally distinct star-forming regions within (or near) the growing Milky Way experienced grossly similar chemical evolution histories. Implications for galaxy formation scenarios are discussed.