Titans metal-poor reference stars. II.

We add 47 giants to the sample of metal-poor dwarfs of Giribaldi et al. (2021A&A...650A.194G, Cat. J/A+A/650/A194), thereby constituting the Titans metal-poor reference stars. Teff was derived by 3D non-LTE H{alpha} modelling, whose accuracy was tested against interferometry and InfraRed Flux Method (IRFM).

Surface gravity (logg) was derived by fitting MgI b triplet lines, whose accuracy was tested against asteroseismology. Metallicity was derived using FeII lines, which was verified to be identical to the [Fe/H] derived from non-LTE spectral synthesis. Teff from 3D non-LTE H{alpha} is equivalent to interferometric and IRFM temperatures within a +/-46K uncertainty.

We achieved precision of ~50K for 34 stars with spectra with the highest S/N. For log g, we achieved a total uncertainty of +/-0.15dex. For [Fe/H], we obtained a total uncertainty of +/-0.09dex. We find that the ionization equilibrium of Fe lines under LTE is not valid in metal-poor giants.

LTE leads to a small but significant metallicity underestimation of ~0.1dex when derived from weak FeI lines, only provided accurate Teff and logg. This bias disappears totally under non-LTE.