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 ≈ 48,000), high signal-to-noise ratio (S/N ≈ 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.