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Context. Tighter constraints on metal-poor stars we observe are needed tobetter understand the chemical processes of the early Universe. Computing astellar spectrum in 3D allows one to model complex stellar behaviours, whichcannot be replicated in 1D. Aims. We examine the effect that the intrinsic CNOabundances have on a 3D model structure and the resulting 3D spectrumsynthesis. Methods. Model atmospheres were computed in 3D for three distinctCNO chemical compositions using the CO 5 BOLD model atmosphere code, and theirinternal structures were examined. Synthetic spectra were computed from thesemodels using Linfor3D and they were compared. New 3D abundance corrections forthe G-band and a selection of UV OH lines were also computed. Results. Thevarying CNO abundances change the metal content of the 3D models. This had aneffect on the model structure and the resulting synthesis. However, it wasfound that the C/O ratio had a larger effect than the overall metal content ofa model. Conclusions. Our results suggest that varying the C/O ratio has asubstantial impact on the internal structure of the 3D model, even in the hotturn-off star models explored here. This suggests that bespoke 3D models, forspecific CNO abundances should be sought. Such effects are not seen in 1D atthese temperature regimes.

astronomy and astrophysics

An in-depth spectroscopic examination of molecular bands from 3D  hydrodynamical model atmospheres. II. Carbon-enhanced metal-poor 3D model  atmospheres