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

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 theta(3D) = 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 theta(1D) = 0.679 +/- 0.007 mas. Coupling the angular diameters with photometry yields effective temperatures with precisions better than 55 K (T-eff = 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-circle dot, R = 24.1 +/- 1.1 R-circle dot and L = 0.213 +/- 0.002 L-circle dot, R = 0.665 +/- 0.014 R-circle dot, 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 alpha (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 T-eff provide stringent constraints for spectroscopic analyses given the sensitivity of abundances to both of these values. The precise determination of T-eff for the two stars brings into question the photometric scales for metal-poor stars.