The Clusters AgeS Experiment (CASE). III. Analysis of the Eccentric Eclipsing Binary V32 in the Globular Cluster NGC 6397

Abstract

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 9.8783d and a large eccentricity of e=0.32. Its systemic velocity (gamma=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 12Gyr and the mass-function derived from the radial velocity curve, we estimate the masses to be M_p=0.79 Msun and M_s=0.23 Msun. The light curve of V32 can be reproduced by adopting R_p=1.569 Rsun and R_s=0.236 Rsun 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.