3D effects on CH lines and the carbon isotopic ratio in ultra metal-poor stars

Metal-poor stars, as relics of the early Universe, preserve valuable information in their spectra. By analyzing their chemical compositions, it is possible to reconstruct aspects of Galactic history and stellar formation. A notable subclass is the carbon-enhanced metal-poor (CEMP) stars, characterised by elevated abundances of carbon relative to iron. Recent studies have used the isotopic ratio 12C/13C, derived from molecular CH features, to investigate the origin of this carbon enhancement, typically relying on one-dimensional (1D) stellar atmosphere models. In this work, the impact of three-dimensional (3D) effects, particularly surface granulation, is quantified by comparing 1D and 3D radiative transfer models. Abundance corrections are derived and applied to observed spectra, evaluating whether conclusions based on 1D models remain robust when accounting for 3D effects.