Oxygen in the early Galaxy: OH lines as tracers of oxygen abundance in extremely metal-poor giant stars

TL;DR

This study investigates how convection affects infrared OH line formation in extremely metal-poor giant stars, revealing that 3D hydrodynamical models provide more accurate oxygen abundances than traditional 1D models, thus improving our understanding of Galactic chemical evolution.

Contribution

It demonstrates the importance of 3D hydrodynamical models in accurately determining oxygen abundance from IR OH lines in EMP giants, addressing discrepancies from previous 1D models.

Findings

Convection significantly strengthens IR OH lines in EMP giants.

3D models yield oxygen abundances consistent with [O I] 630 nm line measurements.

Differences between 3D and 1D models can reach -0.2 to -0.3 dex.

Abstract

Oxygen is a powerful tracer element of Galactic chemical evolution. Unfortunately, only a few oxygen lines are available in the ultraviolet-infrared stellar spectra for the reliable determination of its abundance. Moreover, oxygen abundances obtained using different spectral lines often disagree significantly. In this contribution we therefore investigate whether the inadequate treatment of convection in 1D hydrostatic model atmospheres used in the abundance determinations may be responsible for this disagreement. For this purpose, we used VLT CRIRES spectra of three EMP giants, as well as 3D hydrodynamical CO$^5$BOLD and 1D hydrostatic LHD model atmospheres, to investigate the role of convection in the formation of infrared (IR) OH lines. Our results show that the presence of convection leads to significantly stronger IR OH lines. As a result, the difference in the oxygen abundance determined from IR OH lines with 3D hydrodynamical and classical 1D hydrostatic model atmospheres may reach -0.2 ... -0.3 dex. In case of the three EMP giants studied here, we obtain a good agrement between the 3D LTE oxygen abundances determined by us using vibrational-rotational IR OH lines in the spectral range of 1514-1626 nm, and oxygen abundances determined from forbidden [O I] 630 nm line in previous studies.