The radii of the nearby K5V and K7V stars 61 Cygni A & B

CHARA/FLUOR interferometry and CESAM2k modeling

¹ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France e-mail: Pierre.Kervella@obspm.fr
² Center for High Angular Resolution Astronomy, Georgia State University, PO Box 3965, Atlanta, Georgia 30302-3965, USA
³ Université de Nice-Sophia Antipolis, Lab. Cassiopée, UMR 6202, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice, France
⁴ Department of Physics and Astronomy, Uppsala University, Box 515, 751 20 Uppsala, Sweden
⁵ National Optical Astronomy Observatories, 950 North Cherry Avenue, Tucson, AZ 85719, USA

Received: 28 April 2008
Accepted: 23 June 2008

Abstract

Context. The main sequence binary star 61 Cyg (K5V+K7V) is our nearest stellar neighbour in the northern hemisphere. This proximity makes it a particularly well suited system for very high accuracy interferometric radius measurements.

Aims. Our goal is to constrain the poorly known evolutionary status and age of this bright binary star.

Methods. We obtained high accuracy interferometric observations in the infrared K′ band, using the CHARA/FLUOR instrument. We then computed evolutionary models of 61 Cyg A & B with the CESAM2k code. As model constraints, we used a combination of observational parameters from classical observation methods (photometry, spectroscopy) as well as our new interferometric radii.

Results. The measured limb darkened disk angular diameters are = 1.775 ± 0.013 mas and = 1.581 ± 0.022 mas, respectively for 61 Cyg A and B. Considering the high accuracy parallaxes available, these values translate into photospheric radii of = 0.665 ± 0.005 and = 0.595 ± 0.008 . The new radii constrain efficiently the physical parameters adopted for the modeling of both stars, allowing us to predict asteroseismic frequencies based on our best-fit models.

Conclusions. The CESAM2k evolutionary models indicate an age around 6 Gyr and are compatible with small values of the mixing length parameter. The measurement of asteroseismic oscillation frequencies in 61 Cyg A & B would be of great value to improve the modeling of this important fiducial stellar system, in particular to better constrain the masses.