The radii and limb darkenings of α Centauri A and B

Interferometric measurements with VLTI/PIONIER^⋆

¹ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386), Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
e-mail: pkervell@das.uchile.cl
² LESIA (UMR 8109), Observatoire de Paris, PSL Research University, CNRS, UPMC, Univ. Paris-Diderot, 5 Place Jules Janssen, 92195 Meudon, France
e-mail: pierre.kervella@obspm.fr
³ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange UMR 7293, CS 34229, 06304 Nice Cedex 4, France
⁴ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile

Received: 8 August 2016
Accepted: 11 October 2016

Abstract

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 α 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 θ_LD and the limb darkening parameters of the two solar-type stars in the near-infrared H band (λ = 1.65 μm). We obtain photospheric angular diameters of θ_LD(A) = 8.502 ± 0.038 mas (0.43%) and θ_LD(B) = 5.999 ± 0.025 mas (0.42%), through the adjustment of a power law limb darkening model. We find H band power law exponents of α(A) = 0.1404 ± 0.0050 (3.6%) and α(B) = 0.1545 ± 0.0044 (2.8%), which closely bracket the observed solar value (α_⊙ = 0.15027). Combined with the parallax π = 747.17 ± 0.61 mas previously determined, we derive linear radii of R_A = 1.2234 ± 0.0053 R_⊙ (0.43%) and R_B = 0.8632 ± 0.0037 R_⊙ (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 α Cen A and B provide a robust basis to validate the future evolutions of these models.