1999AJ....117.1771S

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.

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.