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

III. Constraints from a subgiant^⋆

¹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: kfb@phys.au.dk
² Astronomical Observatory, Institute of Theoretical Physics and Astronomy, Vilnius University, Saulėtekio av. 3, 10257 Vilnius, Lithuania
³ Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁵ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁶ Department of Astronomy, University of Wisconsin-Madison, Madison, WI 53706, USA
⁷ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, 1800 Sherman Avenue, Evanston, IL 60201, USA
⁸ Adler Planetarium, Department of Astronomy, 1300 South Lake Shore Drive, Chicago, IL 60605, USA
⁹ Department of Astronomy and Theoretical Physics, Lund Observatory, Lund University, Box 43 221 00 Lund, Sweden
¹⁰ School of Physics, University of New South Wales, NSW 2052, Australia
¹¹ Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, NSW 2006, Australia
¹² Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
¹³ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA

Received: 29 March 2021
Accepted: 25 April 2021

Abstract

Context. Models of stellar structure and evolution can be constrained using accurate measurements of the parameters of eclipsing binary members of open clusters. Multiple binary stars provide the means to tighten the constraints and, in turn, to improve the precision and accuracy of the age estimate of the host cluster. In the previous two papers of this series, we have demonstrated the use of measurements of multiple eclipsing binaries in the old open cluster NGC 6791 to set tighter constraints on the properties of stellar models than was previously possible, thereby improving both the accuracy and precision of the cluster age.

Aims. We identify and measure the properties of a non-eclipsing cluster member, V56, in NGC 6791 and demonstrate how this provides additional model constraints that support and strengthen our previous findings.

Methods. We analyse multi-epoch spectra of V56 from FLAMES in conjunction with the existing photometry and measurements of eclipsing binaries in NGC6971.

Results. The parameters of the V56 components are found to be M_p = 1.103 ± 0.008 M_⊙ and M_s = 0.974 ± 0.007 M_⊙, R_p = 1.764 ± 0.099 R_⊙ and R_s = 1.045 ± 0.057 R_⊙, T_eff,p = 5447 ± 125 K and T_eff,s = 5552 ± 125 K, and surface [Fe/H] = +0.29 ± 0.06 assuming that they have the same abundance.

Conclusions. The derived properties strengthen our previous best estimate of the cluster age of 8.3 ± 0.3 Gyr and the mass of stars on the lower red giant branch (RGB), which is M_RGB = 1.15 ± 0.02 M_⊙ for NGC 6791. These numbers therefore continue to serve as verification points for other methods of age and mass measures, such as asteroseismology.