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

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. HD140283 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 HD140283 (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 theta(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 x 10(-8) erg s(-1) cm(-2),and a maximum of A(V) = 0.1 mag solution of 4.220 +/- 0.067 x 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 +/- 0.08 R-circle dot. Spectroscopic measurements of HD140283 were obtained using HARPS, NARVAL, and UVES and a 1D LTE analysis of Ha 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 HD140283. Once the metallicity has been fixed, the age of the star depends on M, initial helium abundance Y-i, and mixing-length parameter alpha, only two of which are independent. We derive simple equations to estimate one from the other two. We need to adjust a to much lower values than the solar one (similar to 2) in order to fit the observations, and if A(V) = 0.0 mag then 0.5 <= alpha <= 1. We give an equation to estimate t from M, Y-i (alpha), and A(V). Establishing a reference alpha = 1.00 and adopting Y-i = 0.245 we derive a mass and age of HD140283: M = 0.780 +/- 0.010 M-circle dot and t = 13.7 +/- 0.7 Gyr (A(V) = 0.0 mag), or M = 0.805 +/- 0.010 M-circle dot 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.