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 less than or similar to T-ef f less than or similar to 6500 K, 4.0 less than or similar to log g le ss than or similar to 4.6 and overlapping metallicities in the range - 1.3 less than or similar to [Fe/H] less than or similar to -0.5. Equiv alent widths of weak Fe I and Fe II lines are used to determine differ ential values of T-eff and log g. Relative abundances of O, Na, Mg, Si , Ca, Ti, Cr, Fe, Ni, Y and Ba are determined to a precision ranging f rom 0.02 to 0.07 dex. Kinematical data have been collected and used to calculate the stellar orbital parameters, R-max, the maximum distance from the Galactic center, and z(max), the maximum distance from the G alactic plane. A group of 8 halo stars have significantly lower [alpha /Fe] values than disk stars of the same metallicity (alpha = 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 [alpha/Fe] and [N a/Fe] are found for the stars with the largest values of R-max and z(m ax). This may indicate that the anomalous halo stars have been accrete d 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 a bundance ratios in stars are not universal functions of [Fe/H] and tha t the chemical evolution of the Galaxy at [Fe/H] similar or equal to - 1.0 is more complicated than assumed in many models. One halo star, HD 106038, is found to be strongly overabundant in Si, Ni, Y and Ba rela tive to Fe.