Oxygen abundace in the template Halo Giant HD 122563
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HD 122563 is a well-known bright (V ¼ 6:2) halo giant of low metallicity ([Fe/=-2:7). We have observed HD 122563 for infrared OH lines at 1.5–1.7 lm in the H band with the NIRSPEC high-resolution spectrograph at the 10 m Keck Telescope. Optical spectra were obtained with the UVES spectrograph at the 8m VLT UT2 telescope at ESO (Paranal) and the FEROS spectrograph at ESO (La Silla). Based on the optical high-resolution data, a detailed analysis has been carried out, and data on the forbidden [O i] 6300 A ˚ line,unblended by telluric or sky lines, was obtained with the FEROS spectrograph. Signal-to-noise ratios of 200–400 were obtained at resolutions of 37,000 in the H band and 45,000 in the optical. For the analysis we have adopted a photometric eﬀective temperature Teff ¼ 4600 K. Two values for the gravity were adopted: a value deduced from ionization equilibrium, log g ¼ 1:1, with corresponding metallicity [Fe/H ¼ 2:8 and micro-turbulence velocity vt ¼ 2:0kms 1; and log g ¼ 1:5, derived from the Hipparcos parallax, implying[Fe/H ¼ 2:71 and vt ¼ 2:0kms 1. The forbidden [O i] 6300 A ˚ and the permitted O i 7771 A ˚ lines give O/Fe ratios essentially insensitive to model parameter variations, whereas the oxygen abundances from OH lines are sensitive to gravity, giving [O/Fe ¼þ0:9 and +0.7, respectively, for log g ¼ 1:1 and 1.5. We derive the following oxygen abundances: for model 1, [O/Fe ¼þ0:6, +1.1, and +0.9; and for model 2, [O/Fe ¼þ0:6, +1.1, and +0.7, based on the [O i] 6300 A ˚ ,O i 7771 A ˚ , and IR OH 1.6 lm lines, respectively. The diﬀerent oxygen abundance indicators give diﬀerent oxygen abundances, illustrating the problem of oxygen abundance derivation in metal-poor giants. This is important because the age of globular clusters and the production of Li, Be, and B from spallation of C, N, and O atoms in the early Galaxy depend on the oxygen abundance adopted for themetal-poor stars.
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