Benchmark stars for Gaia: fundamental properties of the Population II
  star HD140283 from interferometric, spectroscopic and photometric data
  We determined the fundamental properties of HD 140283 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 0.353 +/- 0.013 milliarcseconds. Using
photometry from the literature we derived the bolometric flux with two
solutions: a zero-reddening one of Fbol = 3.890 +/- 0.066 1E-8 erg/s/cm2 and a
solution with a maximum of Av = 0.1 mag, Fbol= 4.220 +/- 0.067 1E-8 erg/s/cm2.
The interferometric Teff is thus 5534 +/- 103 K or 5647 +/- 105 K and its
radius is R = 2.21 +/- 0.08 Rsol. Spectroscopic measurements of HD140283 were
obtained using HARPS, NARVAL, and UVES and a 1D LTE analysis of H-alpha line
wings yields Teff(Halpha) = 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 Yi and mixing-length parameter alpha, only two of
which are independent. We need to adjust alpha to much lower values than the
solar one (~2) in order to fit the observations, and if Av = 0.0 mag then 0.5 <
alpha < 1. We give an equation to estimate t from M, Yi (alpha) and Av.
Establishing a reference alpha = 1.00 and adopting Yi = 0.245 we derive a mass
and age of HD140283: M = 0.780 +/- 0.010 Msol and t = 13.7 +/- 0.7 Gyr (Av =
0.0) or M = 0.805 +/- 0.010 Msol and t = 12.2 +/- 0.6 Gyr (Av=0.1 mag). Our
stellar models yield an initial metallicity of [Z/X]i = -1.70 and logg = 3.65
+/- 0.03. Asteroseismic observations are critical for overcoming limitations in
our results.
