# Two-dimensional non-LTE \ion{O}{I} 777\,nm line formation in radiation hydrodynamics simulations of Cepheid atmospheres

## TL;DR

This study assesses the accuracy of 1D hydrostatic models for oxygen abundance measurements in Cepheids by comparing them with more realistic 2D non-LTE radiation hydrodynamics simulations, revealing significant phase-dependent differences.

## Contribution

It demonstrates the limitations of 1D LTE models for oxygen line analysis in Cepheids and highlights the importance of phase selection for accurate abundance determination.

## Key findings

- 2D non-LTE vs 1D LTE abundance differences range from -1.0 to -0.25 dex.
- 2D non-LTE vs 1D non-LTE differences range from -0.2 to 0.8 dex.
- Best phases for oxygen abundance analysis are between 0.3 and 0.8, near hydrostatic equilibrium.

## Abstract

Oxygen abundance measurements are important for understanding stellar structure and evolution. Measured in Cepheids, they further provide clues on the metallicity gradient and chemo-dynamical evolution in the Galaxy. However, most of the abundance analyses of Cepheids to date have been based on one-dimensional (1D) hydrostatic model atmospheres. Here, we test the validity of this approach for the key oxygen abundance diagnostic, the \ion{O}{I} $777\,\mathrm{nm}$~triplet lines. We carry out 2D non-LTE radiative transfer clculations across two different 2D radiation hydrodynamics simulations of Cepheid atmospheres, having stellar parameters of $T_\mathrm{eff}= 5600$ K, solar chemical compositions, and $\log\,g= 1.5$ and $2.0$, corresponding to pulsation periods of 9 and 3 days, respectively. We find that the 2D non-LTE versus 1D LTE abundance differences range from $-1.0$~dex to $-0.25$~dex depending on pulsational phase. The 2D non-LTE versus 1D non-LTE abundance differences range from $-0.2$~dex to $0.8$~dex. The abundance differences are smallest when the Cepheid atmospheres are closest to hydrostatic equilibrium, corresponding to phases of around $0.3$ to $0.8$, and we recommend these phases for observers deriving the oxygen abundance from \ion{O}{I} $777\,\mathrm{nm}$ triplet with 1D hydrostatic models.