The radii of the nearby K5V and K7V stars 61 Cyg A & B - CHARA/FLUOR interferometry and CESAM2k modeling

Pierre Kervella (1) , Antoine Mérand (2) , Bernard Pichon (3) , Frédéric Thévenin (3) , Ulrike Heiter (4) , Lionel Bigot (3) , Theo A. ten Brummelaar (2) , Harold A. Mcalister (2) , Stephen T. Ridgway (5) , Nils Turner (2) , Judit Sturmann (2) , Laszlo Sturmann (2) , P. J. Goldfinger (2) , Christopher Farrington (2)

1 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
2 CHARA - Center for High Angular Resolution Astronomy
3 CASSIOPEE - Laboratoire de Cosmologie, Astrophysique Stellaire & Solaire, de Planétologie et de Mécanique des Fluides
4 Uppsala University
5 NOAO - National Optical Astronomy Observatory

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 theta_LD(A) = 1.775 +/- 0.013 mas and theta_LD(B) = 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 R(A) = 0.665 +/- 0.005 Rsun and R(B) = 0.595 +/- 0.008 Rsun. 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 Gyrs 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.

Mots clés: Stars: individual: 61 Cyg; Stars: evolution; Stars: fundamental parameters; Techniques: interferometric

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]

Pierre Kervella, Antoine Mérand, Bernard Pichon, Frédéric Thévenin, Ulrike Heiter, et al.. The radii of the nearby K5V and K7V stars 61 Cyg A & B - CHARA/FLUOR interferometry and CESAM2k modeling. 2008. ⟨hal-00290259⟩