2018ApJ...866...67T

Eclipsing binaries in star clusters offer more stringent tests of stellar evolution theory than field binaries because models must not only match the binary properties, but also the radiative properties of all other cluster members at a single chemical composition and a single age. Here we report new spectroscopic observations of the G-type, detached eclipsing binary EPIC 219394517 in the open cluster Ruprecht 147 ([Fe/H] = +0.10), which was observed in late 2015 by the K2 mission. A joint analysis of our radial-velocity measurements and the K2 light curve shows the 6.5 day orbit to be nearly circular. We derive highly precise masses of 1.0782_–0.0019^+0.0019 M_☉^N and 1.0661_–0.0021^+0.0027 M_☉^N, radii of 1.055 ± 0.011 R_☉^N and 1.042 ± 0.012 R_☉^N, and effective temperatures of 5930 ± 100 K and 5880 ± 100 K for the primary and secondary, respectively. The distance we infer, 283_–16⁺¹⁸ pc, corresponds to a parallax in good agreement with the Gaia/DR2 value for the star. Current stellar evolution models from the MIST and PARSEC series match the above physical properties well at ages of 2.48 and 2.65 Gyr. Isochrones for these same ages and the measured composition, along with our reddening estimate for EPIC 219394517, also show generally good agreement with the optical and near-infrared color-magnitude diagrams of the cluster, which can be constructed with no free parameters as the distances of all member stars are known from Gaia.