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_{eff}_{spec} = 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. Based on observations with the VEGA/CHARA spectrointerferometer.Based on NARVAL and HARPS data obtained within the Gaia DPAC (Data Processing and Analysis Consortium) and coordinated by the GBOG (Ground-Based Observations for Gaia) working group, and on data retrieved from the ESO-ADP database.Full Table 12 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/575/A26