Analysis of the Eccentric Eclipsing Binary V 32 in the Globular Cluster NGC 6397 1

We present spectroscopic and photometric observations of the eclipsing binary V32 located in the central field of the globular cluster NGC 6397. The variable is a single-line spectroscopic binary with an orbital period of 9.8783 d and a large eccentricity of e = 0.32. Its systemic velocity (γ = 20.7 km s) and metallicity ([Fe/H] ∼ -1.9) are both consistent with cluster membership. The primary component of the binary is located at the top of the main-sequence turn-off on the cluster color-magnitude diagram. Only a shallow primary eclipse is observed in the light curve. Based on stellar models for an age of 12 Gyr and the mass-function derived from the radial velocity curve, we estimate the masses to be Mp = 0.79 M⊙ and Ms = 0.23 M⊙. The light curve of V32 can be reproduced by adopting Rp = 1.569 R⊙ and Rs = 0.236 R⊙ for the radii and i = 85.44 deg for the system inclination. The system geometry precludes observations of the secondary eclipse. The large eccentricity of the orbit is puzzling given that for metal poor, halo binaries the transition from circular to eccentric orbit occurs at an orbital period of about 20 days. We suppose that the orbit of V32 was modified relatively recently by dynamical interaction with other cluster star(s). An alternative explanation of the observed eccentricity calls for the presence of a third body in the system. Subject headings: binaries: spectroscopic – stars: individual (V32-NGC 6397) Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland; [email protected] Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101-1292; [email protected] David Dunlap Observatory, Department of Astronomy and Astrophysics, University of Toronto, P.O. Box 360, Richmond Hill, ON L4C 4Y6, Canada; [email protected] Las Campanas Observatory, Casilla 601, La Serena, Chile; [email protected] This paper utilizes data obtained with the 6.5-meter Magellan Telescopes located at Las Campanas Observatory, Chile.