The diameters of (alpha) 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 ESOʹs 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 (theta)UD[A]=8.314 (plus-minus)0.016 mas and (theta)UD[B]= 5.856 (plus-minus)0.027 mas, and limb darkened angular diameters of (theta)LD[A]=8.511 (plus-minus)0.020 mas and (theta)LD[B]=6.001 (plus-minus) 0.034 mas. Combining these values with the parallax from Soderhjelm (1999), we derive linear diameters of D[A]=1.224 (plus-minus) 0.003 D(circled dot operator) and D[B]=0.836 (plus-minus) 0.005 D (circled dot operator). These values are compatible with the masses published by Thevenin et al. (2002) for both stars.