# Carbon and oxygen in metal-poor halo stars

## TL;DR

This study reanalyzed carbon, oxygen, and iron abundances in 39 metal-poor halo stars using advanced 3D non-LTE models, revealing that [C/O] decreases with decreasing [O/H], challenging previous findings.

## Contribution

First to incorporate combined 3D hydrodynamic and non-LTE effects in abundance analysis of metal-poor stars, providing more accurate elemental ratios.

## Key findings

- [C/Fe] remains flat across [Fe/H]
- [O/Fe] increases as [Fe/H] decreases
- [C/O] decreases monotonically with decreasing [O/H]

## Abstract

Carbon and oxygen are key tracers of the Galactic chemical evolution; in particular, a reported upturn in [C/O] towards decreasing [O/H] in metal-poor halo stars could be a signature of nucleosynthesis by massive Population III stars. We reanalyse carbon, oxygen, and iron abundances in thirty-nine metal-poor turn-off stars. For the first time, we take into account three-dimensional (3D) hydrodynamic effects together with departures from local thermodynamic equilibrium (LTE) when determining both the stellar parameters and the elemental abundances, by deriving effective temperatures from 3D non-LTE H$\beta$ profiles, surface gravities from Gaia parallaxes, iron abundances from 3D LTE Feii equivalent widths, and carbon and oxygen abundances from 3D non-LTE Ci and Oi equivalent widths. We find that [C/Fe] stays flat with [Fe/H], whereas [O/Fe] increases linearly up to $0.75$ dex with decreasing [Fe/H] down to $-3.0$ dex. As such [C/O] monotonically decreases towards decreasing [O/H], in contrast to previous findings, mainly by virtue of less severe non-LTE effects for Oi at low [Fe/H] with our improved calculations.