TITANS metal-poor reference stars

II. Red giants and CEMP stars^★,★★

¹ Institut d’Astronomie et d’Astrophysique, Université libre de Bruxelles, CP 226, Boulevard du Triomphe, 1050 Brussels, Belgium
e-mail: riano.giribaldi@ulb.be; rianoesc@gmail.com
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 16 June 2023
Accepted: 17 August 2023

Abstract

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 TITANS metal-poor reference stars.

Methods. We derived T_eff using 3D non-LTE Hα 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 Hα 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.