The chemical compositions of the extreme halo stars HE0107-5240 and HE1327-2326 inferred from 3D hydrodynamical model atmospheres

We investigate the impact of realistic 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE0107-5240 and HE1327-2326. We derive the chemical compositions of the two stars by means of a detailed 3D analysis of spectral lines under the assumption of local thermodynamic equilibrium (LTE). The lower temperatures of the line-forming regions of the hydrodynamical models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by ~ -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundances of iron-peak elements are also decreased but by smaller factors (~ -0.2 dex). We caution however that the neglected non-LTE effects might actually be substantial for these metals. We finally discuss possible implications for studies of early Galactic chemical evolution