2012A&A...545A..17C

We have determined the angular diameters of two metal-poor stars, HD122563 and Gmb1830, using CHARA and Palomar Testbed Interferometer observations. For the giant star HD122563, we derive an angular diameter θ_3D=0.940±0.011-milliarcseconds (mas) using limb-darkening from 3D convection simulations and for the dwarf star Gmb1830 (HD103095) we obtain a 1D limb-darkened angular diameter θ_1D=0.679±0.007mas. Coupling the angular diameters with photometry yields effective temperatures with precisions better than 55K (T_eff=4598±41K and 4818±54K - for the giant and the dwarf star, respectively). Including their distances results in very well-determined luminosities and radii (L=230±7L_☉, R=24.1±1.1R_☉ and L=0.213±0.002L_☉, R=0.665±0.014R_☉, 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 (logg=1.60±0.04, 4.59±0.02dex, respectively). The very small errors on both logg 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.