The diameters of α  Centauri A and B

Abstract

We compare the first direct angular diameter measurements obtained on our closest stellar neighbour, alpha Centauri, to recent model diameters constrained by asteroseismic observations. Using the VINCI instrument installed at ESO's VLT Interferometer (VLTI), the angular diameters of the two main components of the system, alpha Cen A and B, were measured with a relative precision of 0.2% and 0.6% respectively. Particular care has been taken in the calibration of these measurements, considering that VINCI is estimating the fringe visibility using a broadband K filter. We obtain uniform disk angular diameters for alpha Cen A and B of thetaUD [A] = 8.314 +/- 0.016 mas and thetaUD [B] = 5.856 +/- 0.027 mas, and limb darkened angular diameters of thetaLD [A] = 8.511 +/- 0.020 mas and thetaLD [B] = 6.001 +/- 0.034 mas. Combining these values with the parallax from Söderhjelm (\cite{soderhjelm}), we derive linear diameters of D[A] = 1.224 +/- 0.003 Dsun and D[B] = 0.863 +/- 0.005 Dsun. These values are compatible with the masses published by Thévenin et al. (\cite{thevenin02}) for both stars.