Article révisé par les pairs
Résumé : Context. Representative samples of F-, G-, K-type stars located outside of the solar neighbourhood have started to become available in spectroscopic surveys. The fraction of metal-poor ([Fe/H] ≲ −0.8 dex) giants becomes increasingly relevant towards greater distances. In metal-poor stars, effective temperatures ( T eff ) based on local thermodynamic equilibrium (LTE) spectroscopy and on former colour– T eff relations – which are still widely used – have been reported to be inaccurate. It is necessary to recalibrate chemical abundances based on these T eff scales in the multiple available surveys in order to bring them to the same standard scale for their simultaneous use. To this end, a complete sample of standards is required, which until now has been restricted to a few stars with quasi-direct T eff measurements. Aims. We aim to provide a legacy sample of metal-poor standards with proven accurate atmospheric parameters. We add 47 giants to the T ITANS metal-poor reference stars. Methods. We derived T eff using 3D non-LTE H α modelling, the accuracy of which was tested against interferometry and with the Infra Red Flux Method (IRFM). We derived surface gravity (log ɡ ) by fitting Mg I b triplet lines, and tested their accuracy against asteroseismology. Metallicity was derived using Fe  II lines, and we find our results to be identical to the [Fe/H] derived from non-LTE spectral synthesis. Results. The T eff that we find using 3D non-LTE H α is equivalent to interferometric and IRFM temperatures within a ±46 K uncertainty. We achieve precision of ~50 K for 34 stars with spectra with the highest signal-to-noise ratio (S/N). For log ɡ , we achieve a total uncertainty of ±0.15 dex. For [Fe/H], we obtain a total uncertainty of ±0.09 dex. We find that the ionisation equilibrium of Fe lines under LTE is not valid in metal-poor giants. LTE leads to a small but significant metallicity underestimation of ~0.1 dex when derived from weak Fe  I lines, and only provided accurate T eff and log ɡ . This bias totally disappears under non-LTE.

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