Oxygen Abundance in the Template Halo Giant HD 122563

Abstract

HD 122563 is a well-known bright (V ¼ 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 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 8 m VLT UT2 telescope at ESO (Paranal) and the FEROS spectrograph at ESO (La Silla). Based on the opti- cal high-resolution data, a detailed analysis has been carried out, and data on the forbidden (O i) 6300 Aline, 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: 0k m s � 1 ; and log g ¼ 1:5, derived from the Hipparcos parallax, implying (Fe/H �¼� 2:71 and vt ¼ 2: 0k m s � 1 . The forbidden (O i) 6300 Aand the permitted O i 7771 Alines give O/Fe ratios essentially insensitive to model parameter variations, whereas the oxygen abundances from OH lines are sensitive to gravity, giving (O/Fe �¼þ 0:9 and +0.7, respectively, for log g ¼ 1:1 and 1.5. We derive the following oxygen abundances: for model 1, (O/Fe �¼þ 0:6, +1.1, and +0.9; and for model 2, (O/Fe �¼þ 0:6, +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 oxy- gen 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. Subject headings: stars: abundances — stars: individual (HD 122563) — stars: Population II On-line material: machine-readable table