The K2 M67 Study: A Curiously Young Star in an Eclipsing Binary in an Old Open Cluster Based on observations made at Kitt Peak National Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation; with the Tillinghast Reflector Echelle Spectrograph (TRES) on the 1.5 m Tillinghast telescope, located at the Smithsonian Astrophysical Observatory's Fred L.

• Published in: The Astronomical journal Vol. 155; no. 4
• Main Authors: Sandquist, Eric L., Mathieu, Robert D., Quinn, Samuel N., Pollack, Maxwell L., Latham, David W., Brown, Timothy M., Esselstein, Rebecca, Aigrain, Suzanne, Parviainen, Hannu, Vanderburg, Andrew, Stello, Dennis, Somers, Garrett, Pinsonneault, Marc H., Tayar, Jamie, Orosz, Jerome A., Bedin, Luigi R., Libralato, Mattia, Malavolta, Luca, Nardiello, Domenico
• Format: Journal Article
• Language: English
• Published: The American Astronomical Society 16.03.2018
• Subjects: binaries: eclipsing; binaries: spectroscopic; open clusters and associations: individual (M67); stars: distances; stars: low-mass
• DOI: 10.3847/1538-3881/aab0ff
• ISSN: 0004-6256

We present an analysis of a slightly eccentric (e = 0.05), partially eclipsing, long-period (P = 69.73 days) main-sequence binary system (WOCS 12009, Sanders 1247) in the benchmark old open cluster M67. Using Kepler K2 and ground-based photometry, along with a large set of new and reanalyzed spectra, we derived highly precise masses (1.111 0.015 and 0.748 0.005 M ) and radii (1.071 0.008 0.003 and 0.713 0.019 0.026 R , with statistical and systematic error estimates) for the stars. The radius of the secondary star is in agreement with theory. The primary, however, is approximately 15% smaller than reasonable isochrones for the cluster predict. Our best explanation is that the primary star was produced from the merger of two stars, as this can also account for the nondetection of photospheric lithium and its higher temperature relative to other cluster main-sequence stars at the same V magnitude. To understand the dynamical characteristics (low measured rotational line broadening of the primary star and low eccentricity of the current binary orbit), we believe that the most probable (but not the only) explanation is the tidal evolution of a close binary within a primordial triple system (possibly after a period of Kozai-Lidov oscillations), leading to merger approximately 1 Gyr ago. This star appears to be a future blue straggler that is being revealed as the cluster ages and the most massive main-sequence stars die out.