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Abstract

Precise measurements of stellar chemical abundances are the fundamental tool in Galactic Archaeology, and key to unraveling the formation and evolutionary history of the Milky Way Galaxy. Traditional modeling techniques assume 1D hydrostatic stellar atmospheres and local thermodynamic equilibrium (LTE) in the spectrum synthesis, which is increasingly shown to be inadequate. In this thesis, I investigate how the neglect of 3D radiation hydrodynamics (RHD) and non-LTE (NLTE) radiative transfer influences our understanding of Galactic evolution. By deriving NLTE abundances of Ni and Fe for a large stellar sample, I find that LTE trends wrongfully suggest the dominance of classical type Ia supernovae (SNe Ia), while in NLTE, the [Ni/Fe] evolution is best reproduced by ∼ 80 % sub-Chandrasekhar-mass SNe Ia. To overcome these limitations of the current modeling, I present M3DIS, a new, highly efficient, and flexible code for constructing 3D RHD models of stellar atmospheres. M3DIS can relax FGK-type atmospheres in under 5 000 CPU-h, achievable within 24 h on intermediate-size clusters. Using these newly developed tools, I shift the focus towards the most metal-poor stars, which are found to be carbon-enhanced (CEMP stars) in large quantities, and find large 3D effects (up to −0.7 dex) on C abundances. Depending on the metallicity, this reduces the overall CEMP fraction by up to ∼ 30 %, which suggest a 20 % lower contribution from faint supernovae in the early Galaxy. This thesis underlines the need for 3D NLTE modeling in Galactic Archaeology, and provides the tools to make them the new standard in astronomy.