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

¹ Laboratoire Lagrange, UMR 7293, CNRS, Observatoire de la Côte d’Azur, Université de Nice Sophia-Antipolis, Nice, France
e-mail: orlaghc@gmail.com
² Center for High Angular Resolution Astronomy, Georgia State University, PO Box 3965, Atlanta, Georgia 30302-3965, USA
³ Hubble Fellow  
⁴ LESIA-Observatoire de Paris, CNRS UMR 8109, UPMC, Univerité Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁵ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP. 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
⁶ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁷ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁸ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen, Denmark
⁹ Astronomical Observatory/Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
¹⁰ Lowell Observatory, 1400 West Mars Hill Road, Flagstaff, Arizona, 86001, USA

Received: 22 May 2012
Accepted: 19 July 2012

Abstract

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 θ_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 θ_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_⊙, 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 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.