Age and helium content of the open cluster NGC  6791 from multiple eclipsing binary members ^⋆,⋆⋆

II. Age dependencies and new insights

¹ Department of Physics and Astronomy, University of Victoria, PO Box 3055, Victoria, B.C., V8W 3P6, Canada
e-mail: kfb@phys.au.dk
² Department of Physics and Astronomy, Aarhus University, Ny Munkegade, 8000 Aarhus C, Denmark
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Instituto de Astrofisica de Canarias, 38200 La Laguna, Tenerife, Canary Islands, Spain
⁵ Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA
⁶ Dominian Astrophysical Observatory, Herzberg Institute of Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, B.C., V9E 2E7, Canada
⁷ Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
⁸ School of Physics and Astronomy, University of Birmingham, Edgbaston, Birminghma B15 2TT, UK
⁹ Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia
¹⁰ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 8 March 2012
Accepted: 1 May 2012

Abstract

Context. Models of stellar structure and evolution can be constrained by measuring accurate parameters of detached eclipsing binaries in open clusters. Multiple binary stars provide the means to determine helium abundances in these old stellar systems, and in turn, to improve estimates of their age.

Aims. In the first paper of this series, we demonstrated how measurements of multiple eclipsing binaries in the old open cluster NGC 6791 sets tighter constraints on the properties of stellar models than has previously been possible, thereby potentially improving both the accuracy and precision of the cluster age. Here we add additional constraints and perform an extensive model comparison to determine the best estimates of the cluster age and helium content, employing as many observational constraints as possible.

Methods. We improve our photometry and correct empirically for differential reddening effects. We then perform an extensive comparison of the new colour-magnitude diagrams (CMDs) and eclipsing binary measurements to Victoria and DSEP isochrones in order to estimate cluster parameters. We also reanalyse a spectrum of the star 2–17 to improve [Fe/H] constraints.

Results. We find a best estimate of the age of  ~8.3 Gyr for NGC 6791 while demonstrating that remaining age uncertainty is dominated by uncertainties in the CNO abundances. The helium mass fraction is well constrained at Y = 0.30±0.01 resulting in ΔY/ΔZ ~  1.4 assuming that such a relation exists. During the analysis we firmly identify blue straggler stars, including the star 2–17, and find indications for the presence of their evolved counterparts. Our analysis supports the RGB mass-loss found from asteroseismology and we determine precisely the absolute mass of stars on the lower RGB, M_RGB = 1.15±0.02 M_⊙. This will be an important consistency check for the detailed asteroseismology of cluster stars.

Conclusions. Using multiple, detached eclipsing binaries for determining stellar cluster ages, it is now possible to constrain parameters of stellar models, notably the helium content, which were previously out of reach. By observing a suitable number of detached eclipsing binaries in several open clusters, it will be possible to calibrate the age-scale and the helium enrichment parameter Δ Y/Δ Z, and provide firm constraints that stellar models must reproduce.