Benchmark stars for Gaia Fundamental properties of the Population II star HD 140283 from interferometric, spectroscopic, and photometric data^{⋆,⋆⋆,⋆⋆⋆}

¹ Institut d’Astrophysique Spatiale, Université Paris XI, UMR 8617, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
e-mail: orlagh.creevey@ias.u-psud.fr
² Laboratoire Lagrange, Université de Nice Sophia-Antipolis, UMR 7293, CNRS, Observatoire de la Côte d’Azur, Nice, France
³ Institutionen för fysik och astronomi, Uppsala universitet, Box 516, 751 20 Uppsala, Sweden
⁴ Max-Planck-Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁵ Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001, USA
⁶ Department of Astronomy, Yale University, New Haven, CT 06511, USA
⁷ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, PSL, 5 place Jules Janssen, 92195 Meudon, France
⁸ UMI-FCA, CNRS/INSU, France (UMI 3386), and Dept. de Astronomía, Universidad de Chile, Santiago, Chile
⁹ Université Grenoble Alpes, IPAG, 38000 Grenoble, France
¹⁰ CNRS, IPAG, 38000 Grenoble, France
¹¹ Georgia State University, PO Box 3969, Atlanta GA 30302-3969, USA
¹² CHARA Array, Mount Wilson Observatory, 91023 Mount Wilson CA, USA

Received: 30 May 2014
Accepted: 20 November 2014

Abstract

Metal-poor halo stars are important astrophysical laboratories that allow us to unravel details about many aspects of astrophysics, including the chemical conditions at the formation of our Galaxy, understanding the processes of diffusion in stellar interiors, and determining precise effective temperatures and calibration of colour-effective temperature relations. To address any of these issues the fundamental properties of the stars must first be determined. HD 140283 is the closest and brightest metal-poor Population II halo star (distance = 58 pc and V = 7.21), an ideal target that allows us to approach these questions, and one of a list of 34 benchmark stars defined for Gaia astrophysical parameter calibration. In the framework of characterizing these benchmark stars, we determined the fundamental properties of HD 140283 (radius, mass, age, and effective temperature) by obtaining new interferometric and spectroscopic measurements and combining them with photometry from the literature. The interferometric measurements were obtained using the visible interferometer VEGA on the CHARA array and we determined a 1D limb-darkened angular diameter of θ_1D = 0.353 ± 0.013 milliarcsec. Using photometry from the literature we derived the bolometric flux in two ways: a zero reddening solution (A_V = 0.0 mag) of F_bol of 3.890 ± 0.066 × 10^-8 erg s^-1 cm^-2, and a maximum of A_V = 0.1 mag solution of 4.220 ± 0.067 × 10^-8 erg s^-1 cm^-2. The interferometric T_eff is thus between 5534 ± 103 K and 5647 ± 105 K and its radius is R = 2.21 ± 0.08R_⊙. Spectroscopic measurements of HD 140283 were obtained using HARPS, NARVAL, and UVES and a 1D LTE analysis of Hα line wings yielded T_effspec = 5626 ± 75 K. Using fine-tuned stellar models including diffusion of elements we then determined the mass M and age t of HD 140283. Once the metallicity has been fixed, the age of the star depends on M, initial helium abundance Y_i, andmixing-length parameter α, only two of which are independent. We derive simple equations to estimate one from the other two. We need to adjust α to much lower values than the solar one (~2) in order to fit the observations, and if A_V = 0.0 mag then 0.5 ≤ α ≤ 1. We give an equation to estimate t from M, Y_i (α), and A_V. Establishing a reference α = 1.00 and adopting Y_i = 0.245 we derive a mass and age of HD 140283: M = 0.780 ± 0.010M_⊙ and t = 13.7 ± 0.7 Gyr (A_V = 0.0 mag), or M = 0.805 ± 0.010M_⊙ and t = 12.2 ± 0.6 Gyr (A_V = 0.1 mag). Our stellar models yield an initial (interior) metal-hydrogen mass fraction of [ Z/X ] _i = −1.70 and log g = 3.65 ± 0.03. Theoretical advances allowing us to impose the mixing-length parameter would greatly improve the redundancy between M,Y_i, and age, while from an observational point of view, accurate determinations of extinction along with asteroseismic observations would provide critical information allowing us to overcome the current limitations in our results.