The diameters of α Centauri A and B -: A comparison of the asteroseismic and VINCI/VLTI views

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 alphaCen A and B of theta(UD)[A] = 8.314+/-0.016 mas and theta(UD)[B] = 5.856+/-0.027 mas, and limb darkened angular diameters of theta(LD)[A] = 8.511+/-0.020 mas and theta(LD)[B] = 6.001+/-0.034 mas. Combining these values with the parallax from Soderhjelm (1999), we derive linear diameters of D[A] = 1.224+/-0.003 D-circle dot and D[B] = 0.863+/-0.005 D-circle dot. These values are compatible with the masses published by Thevenin et al. (2002) for both stars.