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Context. Although oxygen is an important tracer of Galactic chemicalevolution, measurements of its abundance in the atmospheres of the oldestGalactic stars are still scarce and rather imprecise. At the lowest end of themetallicity scale, oxygen can only be measured in giant stars and in most ofcases such measurements rely on a single forbidden [O I] 630 nm line that isvery weak and frequently blended with telluric lines. Although molecular OHlines located in the ultraviolet and infrared could also be used for thediagnostics, oxygen abundances obtained from the OH lines and the [O I] 630 nmline are usually discrepant to a level of ~0.3-0.4 dex. Aims. We study theinfluence of convection on the formation of the infrared (IR) OH lines and theforbidden [O I] 630 nm line in the atmospheres of extremely metal-poor (EMP)red giant stars. Methods. We used high-resolution and high signal-to-noiseratio spectra of four EMP red giant stars obtained with the VLT CRIRESspectrograph. For each EMP star, 4-14 IR OH vibrational-rotational lineslocated in the spectral range of 1514-1548 and 1595-1632 nm were used todetermine oxygen abundances by employing standard 1D LTE abundance analysismethodology. We then corrected the 1D LTE abundances obtained from eachindividual OH line for the 3D hydrodynamical effects. Results. We find that theinfluence of convection on the formation of [O I] 630 nm line in theatmospheres of EMP giants studied here is minor, which leads to very small3D-1D abundance corrections (< -0.01 dex). On the contrary, IR OH lines arestrongly affected by convection and thus the abundance corrections for theselines are significant, 3D-1D ~ -0.2 ... -0.3 dex. These abundance correctionsdo indeed bring the 1D LTE oxygen abundances of EMP red giants obtained usingIR OH lines into better agreement with those determined from the [O I] 630 nmline.

astronomy and astrophysics

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant  stars. IV. Oxygen diagnostics in extremely metal-poor red giants with  infrared OH lines