Alpha Element Populations Among Local Halo Stars

Abstract

Previous studies have shown that the halo of the Milky Way galaxy is made up of two distinct stellar populations, one from dissipative collapse and the other accreted. Elemental abundances with small relative uncertainties along with kinematics are determined for 20 local halo stars in the metallicity range −2.2 ≤ [Fe/H] ≤−1.2. Stars with metallicities [Fe/H] > −1.75 show clear separation into high-α and low-α groups. New results extend the work of Nissen & Schuster to the elements Co and K and to lower metallicities. The five program stars with [Fe/H] < −1.75 appear to follow the low-α sequence and may be distinguishable by lower [Ba/Fe] abundances. The results for potassium help to clarify its behavior for −2.2 ≤ [Fe/H] ≤ −1.2 with [K/Fe] ∼ 0.25 and approximately constant with [Fe/H]. Evidence is discussed regarding the cause of the low [α/Fe] abundances, i.e., whether a lower star-formation rate resulted in slower chemical evolution or if the initial mass function was deficient in high-mass stars. The low-α stars show larger dispersions in U and W velocities, as expected for an accreted population, but unlike Nissen & Schuster we find that the high-α and low-α stars do not have significantly different net orbital rotation (V).