CHEMICAL-COMPOSITION AND ATMOSPHERIC PARAMETERS OF METAL-POOR HALO STARS

The abundance of oxygen in 9 metal-poor stars is derived from high res olution observations of OH lines in the spectral region 3138-3155 angs trom. The same spectra were previously used to determine the beryllium abundance (Gilmore et al. 1992). In addition, Mg, Ca, Ti, Cr, and Fe abundances are derived from accurate equivalent widths of weak atomic absorption lines in the 5100-6100 angstrom spectral region. Atmospheri c parameters, T(eff) and g, are determined from the Stromgren photomet ric indices b - y and c1. New accurate oscillator strengths for the Fe i and Fe II lines allow an important check of these parameters as wel l as the assumption of LTE; the Fe i excitation balance is consistent with the photometric temperatures, whereas the Fe II/Fe I ionization b alance suggests an over-ionization of iron amounting to about 0.15 dex with respect to LTE. The derived abundance ratios between the alpha-e lements and Fe are nearly constant in the metallicity range -3.2 less than or similar [Fe/H] less than or similar -1.8. Any possible scatter of Mg/Fe, Ca/Fe and Ti/Fe in the galactic halo is less than 0.06 dex and an upper limit for the scatter of O/Fe is 0. 15 dex. This suggests that the IMF was similar in different regions of the inner halo and t hat the mixing of nucleosynthesis products was very efficient in galac tic regions of typically globular cluster masses.