Titans metal-poor reference stars II. Red giants and CEMP stars

Representative samples of F-, G-, K-type stars located out of the Solar Neighbourhood has started to be available in spectroscopic surveys. The fraction of metal-poor ([Fe/H]~\(\lesssim -0.8\)~dex) giants becomes increasingly relevant to far distances. In metal-poor stars, effective temperatures (\(T_{\mathrm{eff}}\)) based on LTE spectroscopy and on former colour-\(T_{\mathrm{eff}}\) relations of still wide use have been reported to be inaccurate. It is necessary to re-calibrate chemical abundances based on these \(T_{\mathrm{eff}}\) scales in the multiple available surveys to bring them to the same standard scale for their simultaneous use. For that, a complete sample of standards is required, which so far, is restricted to a few stars with quasi-direct \(T_{\mathrm{eff}}\) measurements. We aim at providing a legacy sample of metal-poor standards with proven accurate atmospheric parameters. We add 47 giants to the sample of metal-poor dwarfs of Giribaldi et al. 2021, thereby constituting the Titans metal-poor reference stars. \(T_{\mathrm{eff}}\) was derived by 3D non-LTE H\(\alpha\) modelling, whose accuracy was tested against interferometry and InfraRed Flux Method (IRFM). Surface gravity (log \(g\)) was derived by fitting Mg~I~b triplet lines, whose accuracy was tested against asteroseismology. Metallicity was derived using Fe II lines, which was verified to be identical to the [Fe/H] derived from non-LTE spectral synthesis. \(T_{\mathrm{eff}}\) from 3D non-LTE H\(\alpha\) is equivalent to interferometric and IRFM temperatures within a \(\pm\)46~K uncertainty. We achieved precision of \(\sim\)50~K for 34 stars with spectra with the highest S/N. For log \(g\), we achieved a total uncertainty of \(\pm\)0.15~dex. For [Fe/H], we obtained a total uncertainty of \(\pm\)0.09~dex. We find that the ionization equilibrium of Fe lines under LTE is not valid in metal-poor giants.