The radii and limb darkenings of α Centauri A and B: Interferometric measurements with VLTI/PIONIER

The photospheric radius is one of the fundamental parameters governing the radiative equilibrium of a star. We report new observations of the nearest solar-type stars alpha Centauri A (G2V) and B (K1V) with the VLTI/PIONIER optical interferometer. The combination of four configurations of the VLTI enable us to measure simultaneously the limb darkened angular diameter [theta] sub(LD) and the limb darkening parameters of the two solar-type stars in the near-infrared H band ([lambda]= 1.65 mu m). We obtain photospheric angular diameters of [theta] sub(LD)(A) = 8.502 + or - 0.038mas (0.43%) and [theta] sub(LD)(B) = 5.999 + or - 0.025mas (0.42%), through the adjustment of a power law limb darkening model. We find H band power law exponents of alpha (A) = 0.1404 + or - 0.0050 (3.6%) and alpha (B) = 0.1545 + or - 0.0044 (2.8%), which closely bracket the observed solar value ( alpha sub([middot in circle])= 0.15027). Combined with the parallax [pi]= 747.17 + or - 0.61mas previously determined, we derive linear radii of R sub(A)= 1.2234 + or - 0.0053 R sub([middot in circle])(0.43%) and R sub(B)= 0.8632 + or - 0.0037 R sub([middot in circle])(0.43%). The power law exponents that we derive for the two stars indicate a significantly weaker limb darkening than predicted by both 1D and 3D stellar atmosphere models. As this discrepancy is also observed on the near-infrared limb darkening profile of the Sun, an improvement of the calibration of stellar atmosphere models is clearly needed. The reported PIONIER visibility measurements of alpha Cen A and B provide a robust basis to validate the future evolutions of these models.