# A Differential Abundance Analysis of Very Metal-Poor Stars

## TL;DR

This study performs a detailed differential chemical abundance analysis of nine very metal-poor main sequence halo stars, revealing typical alpha-element enhancement and highlighting discrepancies between spectroscopic and photometric parameters.

## Contribution

It provides a line-by-line differential abundance analysis of very metal-poor stars and discusses the limitations of current atmospheric models in explaining observed properties.

## Key findings

- Average [$
m{ ext{alpha}}$/Fe] = 0.34 dex, typical of the Milky Way
- Significant disagreement between spectroscopic and photometric parameters
- Potential need for fully 3D non-LTE models to resolve ionization imbalance

## Abstract

We have performed a differential, line-by-line, chemical abundance analysis, ultimately relative to the Sun, of nine very metal-poor main sequence halo stars, near [Fe/H]=$-$2 dex. Our abundances range from $-2.66\leq\mathrm{[Fe/H]}\leq-1.40$ dex with conservative uncertainties of 0.07 dex. We find an average [$\alpha$/Fe]$=0.34\pm0.09$ dex, typical of the Milky Way. While our spectroscopic atmosphere parameters provide good agreement with HST parallaxes, there is significant disagreement with temperature and gravity parameters indicated by observed colors and theoretical isochrones. Although a systematic underestimate of the stellar temperature by a few hundred degrees could explain this difference, it is not supported by current effective temperature studies and would create large uncertainties in the abundance determinations. Both 1D and $\langle$3D$\rangle$ hydrodynamical models combined with separate 1D non-LTE effects do not yet account for the atmospheres of real metal-poor MS stars, but a fully 3D non-LTE treatment may be able to explain the ionization imbalance found in this work.