Oxygen Abundance in the Template Halo Giant HD 122563

Abstract

HD 122563 is a well-known bright (V=6.2) halo giant of low metallicity ([Fe/H]~-2.7). We have observed HD 122563 for infrared OH lines at 1.5-1.7 μm 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 8 m 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 Å 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 T_eff=4600 K. Two values for the gravity were adopted: a value deduced from ionization equilibrium, logg=1.1, with corresponding metallicity [Fe/H]=-2.8 and microturbulence velocity v_t=2.0 km s^-1; and logg=1.5, derived from the Hipparcos parallax, implying [Fe/H]=-2.71 and v_t=2.0 km s^-1. The forbidden [O I] 6300 Å and the permitted O I 7771 Å 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]=+0.9 and +0.7, respectively, for logg=1.1 and 1.5. We derive the following oxygen abundances: for model 1, [O/Fe]=+0.6, +1.1, and +0.9; and for model 2, [O/Fe]=+0.6, +1.1, and +0.7, based on the [O I] 6300 Å, O I 7771 Å, and IR OH 1.6 μm 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 the metal-poor stars.

Observations carried out with the Keck Telescope, Mauna Kea, Hawaii, within the Gemini-Keck agreement, the Very Large Telescope at the European Southern Observatory (ESO), Paranal, Chile, and the 1.5 m ESO telescope at ESO, La Silla, Chile.