Fundamental properties of the Population II fiducial stars HD?122563 and Gmb?1830 from CHARA interferometric observations

摘要

We have determined the angular diameters of two metal-poor stars, HD?122563 and?Gmb?1830, using CHARA and Palomar Testbed Interferometer observations. For the giant star HD?122563, we derive an angular diameter θ3D?=?0.940?±?0.011 milliarcseconds (mas) using limb-darkening from?3D convection simulations and for the dwarf star?Gmb?1830 (HD?103095) we obtain a 1D limb-darkened angular diameter θ1D?=?0.679?±?0.007?mas. Coupling the angular diameters with photometry yields effective temperatures with precisions better than?55?K (Teff?= 4598 ?±? 41?K and 4818 ?±? 54?K?– for the giant and the dwarf star, respectively). Including their distances results in very well-determined luminosities and radii (L?=?230?±?7?L⊙, R?=?24.1?±?1.1?R⊙ and L?=?0.213?±?0.002?L⊙, R?=?0.665?±?0.014?R⊙, respectively). We used the?CESAM2k stellar structure and evolution code in order to produce models that fit the observational data. We found values of the mixing-length parameter?α (which describes?1D convection) that depend on the mass of the star. The masses were determined from the models with precisions of??<3% and with the well-measured radii excellent constraints on the surface gravity are obtained (log?g?=?1.60?±?0.04, 4.59?±?0.02?dex, respectively). The very small errors on both?log?g and?Teff provide stringent constraints for spectroscopic analyses given the sensitivity of abundances to both of these values. The precise determination of?Teff for the two stars brings into question the photometric scales for metal-poor stars.