CHEMICAL COMPOSITION OF HALO AND DISK STARS WITH OVERLAPPING METALLICITIES

High resolution (R = 60000), 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 Fei and Feii lines are used to determine dif- ferential values of Teff and logg. Relative abundances of O, Na, Mg,Si,Ca,Ti,Cr,Fe,Ni,YandBaaredeterminedtoaprecision ranging from 0.02 to 0.07 dex. Kinematical data have been col- lected 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. Agroupof8halostarshavesignicantlylower(/Fe)values thandiskstarsofthesamemetallicity( O,Mg,Si,CaorTi). These stars are also underabundant in Na and Ni and maybe in Cr.(Na/Fe)rangesfrom 0:4to+0.1,and(Ni/Fe)issurprisingly well correlated with (Na/Fe). The smallest values of (/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 theGalaxyat(Fe/H) ' 1 :0ismorecomplicatedthanassumed in many models. Onehalostar,HD 106038,isfoundtobestronglyoverabun- dant in Si, Ni, Y and Ba relative to Fe.