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Recent developments in the three-dimensional (3D) spectral synthesis codeLinfor3D have meant that, for the first time, large spectral wavelengthregions, such as molecular bands, can be synthesised with it in a short amountof time. A detailed spectral analysis of the synthetic G-band for several dwarfturn-off-type 3D atmospheres (5850 <= T_eff [K] <= 6550, 4.0 <= log g <= 4.5,-3.0 <= [Fe/H] <= -1.0) was conducted, under the assumption of localthermodynamic equilibrium. We also examine carbon and oxygen molecule formationat various metallicity regimes and discuss the impact it has on the G-band.Using a qualitative approach, we describe the different behaviours between the3D atmospheres and the traditional one-dimensional (1D) atmospheres and how thedifferent physics involved inevitably leads to abundance corrections, whichdiffer over varying metallicities. Spectra computed in 1D were fit to every 3Dspectrum to determine the 3D abundance correction. Early analysis revealed thatthe CH molecules that make up the G-band exhibited an oxygen abundancedependency; a higher oxygen abundance leads to weaker CH features. Nitrogenabundances showed zero impact to CH formation. The 3D corrections are alsostronger at lower metallicity. Analysis of the 3D corrections to the G-bandallows us to assign estimations of the 3D abundance correction to most dwarfstars presented in the literature. The 3D corrections suggest that A(C) in CEMPstars with high A(C) would remain unchanged, but would decrease in CEMP starswith lower A(C). It was found that the C/O ratio is an important parameter tothe G-band in 3D. Additional testing confirmed that the C/O ratio is an equallyimportant parameter for OH transitions under 3D. This presents a clearinterrelation between the carbon and oxygen abundances in 3D atmospheresthrough their molecular species, which is not seen in 1D.

astronomy and astrophysics

An in-depth spectroscopic examination of molecular bands from 3D  hydrodynamical model atmospheres I. Formation of the G-band in metal-poor  dwarf stars