2017A&A...599A.128P

Context. Although oxygen is an important tracer of the early Galactic evolution, its abundance trends with metallicity are still relatively poorly known at [Fe/H]≤-2.5. This is in part due to a lack of reliable oxygen abundance indicators in the metal-poor stars, and in part due to shortcomings in 1D LTE abundance analyses where different abundance indicators, such as OH lines located in the UV andIR or the forbidden [OI] line at 630nm, frequently provide inconsistent results. Aims.In this study, we determined the oxygen abundance in the metal-poor halo giant HD 122563 using a 3D hydrodynamical CO⁵BOLD model atmosphere. Our main goal was to understand whether a 3D LTE analysis can help to improve the reliability of oxygen abundances that are determined from OH UV lines in comparison to those obtained using standard 1D LTE methodology.
Methods. The oxygen abundance in HD 122563 was determined using 71 OH UV lines located in the wavelength range between 308-330nm. The analysis was performed using a high-resolution VLT UVES spectrum with a 1D LTE spectral line synthesis performed using the SYNTHE package and classical ATLAS9 model atmosphere. Subsequently, a 3D hydrodynamical CO⁵BOLD and 1D hydrostatic LHD model atmospheres were used to compute 3D-1D abundance corrections. For this, the microturbulence velocity used with the 1D LHD model atmosphere was derived from the hydrodynamical CO⁵BOLD model atmosphere of HD 122563. The obtained abundance corrections were then applied to determine 3D LTE oxygen abundances from each individual OH UV line.
Results. As in previous studies, we found trends in the 1D LTE oxygen abundances determined from OH UV lines with line parameters, such as the line excitation potential, χ, and the line equivalent width, W. These trends become significantly less pronounced in 3D LTE. Using OH UV lines, we determined a 3D LTE oxygen abundance in HD 122563 of A(O)_3DLTE=6.23±0.13 ([O/Fe]=0.07±0.13). This is in fair agreement with the oxygen abundance obtained from OHIR lines, A(O)_3DLTE=6.39±0.11 ([O/Fe]=0.23±0.11), but it is noticeably lower than that determined when using the forbidden [OI] line, A(O)_3DLTE=6.53±0.15 ([O/Fe]=0.37±0.15). While the exact cause of this discrepancy remains unclear, it is very likely that non-LTE effects may play a decisive role here. Oxygen-to-iron ratios determined in HD 122563 using OH UV/IR lines and the forbidden [OI] line fall on the lower boundary of the [O/Fe] distribution as observed in the Galactic field stars at this metallicity and suggest a very mild oxygen overabundance with respect to iron, [O/Fe]≤0.4.