Molecular line formation in the extremely metal-poor star BD+44 493

TL;DR

This study analyzes molecular absorption lines in the extremely metal-poor star BD+44 493, revealing that 1D LTE models do not fully capture the star's atmospheric conditions, supporting the need for 3D modeling.

Contribution

It provides observational evidence of the limitations of 1D LTE models in describing the atmospheres of metal-poor stars, highlighting the importance of 3D models.

Findings

Significant excitation potential dependence in molecular lines

Weak trend in neutral iron lines compared to molecular lines

Supports presence of cool atmospheric layers not modeled in 1D

Abstract

Molecular absorption lines of OH (99 lines) and CH (105 lines) are measured for the carbon-enhanced metal-poor star BD+44 493 with [Fe/H]=-3.8. The abundances of oxygen and carbon determined from individual lines based on an 1D-LTE analysis exhibit significant dependence on excitation potentials of the lines; d log e/d chi ~ -0.15 - -0.2 dex/eV, where e and chi are elemental abundances from individual spectral lines and their excitation potentials, respectively. The dependence is not explained by the uncertainties of stellar parameters, but suggests that the atmosphere of this object possesses a cool layer that is not reproduced by the 1D model atmosphere. This result agrees with the predictions by 3D model calculations. Although absorption lines of neutral iron exhibit similar trend, it is much weaker than found in molecular lines and that predicted by 3D LTE models.