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Several large stellar spectroscopic surveys are producing overwhelmingamounts of data that can be used for determining stellar atmospheric parametersand chemical abundances. Nonetheless, the accuracy achieved in the derivedastrophysical parameters is still insufficient, mainly because of the paucityof adequate calibrators, particularly in the metal-poor regime ([Fe/H] $\leq-$1.0). Here, we introduce the Titans metal-poor reference stars: a sample of41 dwarf and subgiant stars with accurate parameters. Effective temperatures(Teff) were derived by fitting observed H$\alpha$ profiles with synthetic linescomputed using 3D hydrodynamic NLTE models. Surface gravities (logg) werecomputed using evolutionary tracks and parallaxes from Gaia EDR3. The samemethods recover the Teff values of the Gaia benchmark stars, which are mostlybased on interferometric measurements, with a 1$\sigma$ dispersion of $\pm 50$K. We assume this to be the accuracy of the H$\alpha$ profiles computed from 3Dnon-LTE models for metal-poor dwarfs and subgiants. We achieved an internalprecision typically between 30-40 K, these errors dominated by instrumentaleffects. The final total uncertainty for the Teff values of the Titans are thusestimated to be of the order of $1\%$. The typical error for logg is $\leq$0.04 dex. In addition, we identified a few members of Gaia-Enceladus, ofSequoia, and of the Helmi stream in our sample. These stars can pave the wayfor the accurate chemical characterization of these Galactic substructures.Using the Titans as reference, large stellar surveys will be able to improvethe internal calibration of their astrophysical parameters. Ultimately, thissample will help users of data from Gaia and large surveys in reaching theirgoal of redefining our understanding of stars, stellar systems, and the MilkyWay.

astronomy and astrophysics

Titans metal-poor reference stars. I. Accurate effective temperatures  and surface gravities for dwarfs and subgiants from 3D non-LTE H$\alpha$  profiles and Gaia parallaxes