A new standard: age and distance for the open cluster NGC 6791 from the eclipsing binary member V20 ^*,**

¹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade, 8000 Aarhus C, Denmark e-mail: fgj@phys.au.dk
² Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark

Received: 5 August 2008
Accepted: 30 September 2008

Abstract

Context. We wish to determine accurate ages for open clusters and use this, in conjunction with colour–magnitude diagrams, to constrain models of stellar structure and evolution.

Aims. The detached eclipsing binary V20 in the old, metal-rich ( = +0.40) open cluster NGC 6791 is studied in order to determine highly accurate masses and radii of its components. This allows the cluster age to be established with high precision, using isochrones in the mass-radius diagram.

Methods. We employ high-resolution UVES spectroscopy of V20 to determine the spectroscopic orbit and time-series photometry to obtain the photometric elements.

Results. The masses and radii of the V20 components are found to be and (primary) and and (secondary). The primary is located almost exactly at the hottest point along the cluster isochrone, and the secondary is a ~7 times fainter main-sequence star. We determine an apparent cluster distance-modulus of (average of primary and secondary). The cluster age is obtained from comparisons with theoretical isochrones in the mass-radius diagram. Using the isochrones from Victoria-Regina with = +0.37 we find Gyr, whereas the Yonsei-Yale (Y²) isochrones lead to Gyr, and BaSTI isochrones to Gyr. In a mass-radius diagram, the 7.7 Gyr VRSS and 9.0 Gyr BaSTI isochrones overlap nearly perfectly despite the age-difference. This model dependence, which is significantly larger than the precision determined from mass, radius, and abundance uncertainties, prevents a definitive age-determination of the cluster.

Conclusions. Using detached eclipsing binaries for determination of cluster ages, the dominant error is due to differences among stellar models and no longer to observational errors in cluster reddening and distance. By observing a suitable number of detached eclipsing binaries in several open clusters it should be possible to calibrate the age-scale and provide firm constraints which stellar models must reproduce.