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We have determined the angular diameters of two metal-poor stars, HD 122563and Gmb 1830, using CHARA and Palomar Testbed Interferometer observations. Forthe giant star HD 122563, we derive an angular diameter theta_3D = 0.940 +-0.011 milliarcseconds (mas) using limb-darkening from 3D convection simulationsand for the dwarf star Gmb 1830 (HD 103095) we obtain a 1D limb-darkenedangular diameter theta_1D = 0.679 +- 0.007 mas. Coupling the angular diameterswith 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-determinedluminosities and radii (L = 230 +- 6 L_sun, R = 23.9 +- 1.9 R_sun and L = 0.213+- 0.002 L_sun, R = 0.664 +- 0.015 R_sun, respectively). We used the CESAM2kstellar structure and evolution code in order to produce models that fit theobservational data. We found values of the mixing-length parameter alpha (whichdescribes 1D convection) that depend on the mass of the star. The masses weredetermined from the models with precisions of <3% and with the well-measuredradii excellent constraints on the surface gravity are obtained (log g = 1.60+- 0.04, 4.59 +- 0.02, respectively). The very small errors on both log g andTeff provide stringent constraints for spectroscopic analyses given thesensitivity of abundances to both of these values. The precise determination ofTeff for the two stars brings into question the photometric scales formetal-poor stars.

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