Chemical composition of halo and disk stars with overlapping metallicities

High resolution (R = 60 000), high S/N spectra have been obtained for 13 halo stars and 16 disk stars with 5400 <∼ Teff <∼ 6500 K, 4.0 <∼ logg <∼ 4.6 and overlapping metallicities in the range −1.3 <∼ [Fe/H] <∼ −0.5. Equivalent widths of weak Fe i and Fe ii lines are used to determine differential values of Teff and logg. Relative abundances of O, Na, Mg, Si, Ca, Ti, Cr, Fe, Ni, Y andBa are determined to a precision ranging from 0.02 to 0.07 dex. Kinematical data have been collected and used to calculate the stellar orbital parameters,Rmax, the maximum distance from the Galactic center, and zmax, the maximum distance from the Galactic plane. Agroupof 8halo stars have significantly lower [α/Fe] values than disk stars of the samemetallicity (α ≡O,Mg, Si, Ca or Ti). These stars are also underabundant in Na and Ni and maybe in Cr. [Na/Fe] ranges from−0.4 to+0.1, and [Ni/Fe] is surprisingly well correlated with [Na/Fe]. The smallest values of [α/Fe] and [Na/Fe] are found for the stars with the largest values of Rmax and zmax. This may indicate that the anomalous halo stars have been accreted from dwarf galaxies with a chemical evolution history different from that of the inner halo and the disk. In any case the data show that abundance ratios in stars are not universal functions of [Fe/H] and that the chemical evolution of theGalaxy at [Fe/H] ' −1.0 ismore complicated than assumed in many models. One halo star, HD 106038, is found to be strongly overabundant in Si, Ni, Y and Ba relative to Fe.