The Blue Straggler RS CVn Star S1082 in M67: A Detailed Light Curve and the Possibility of a Triple

Abstract

We present a picture of the very unusual blue straggler S1082 in the old open cluster M67 whose light is the sum of a close binary (P = 1.0677978 d) and another cluster member. We provide a complete V-band light curve for the system, and show a number of unusual features including brightness variations at the 0.01-0.03 mag level from month to month at all phases, and a narrow primary and broad secondary eclipses. We use spectra from several sources to constrain the temperatures of the three known components, the relative flux contributions, rotational velocities, and radial velocities. The data clearly show that the brightest star (narrow-line component) seen in the spectrum is on an orbit with period P = 1189 +/- 7 d and eccentricity e = 0.57 +/- 0.08, although we cannot prove that there is a dynamical link between it and the close binary. Our models of the system indicate that the cooler component of the close binary lies on the main sequence near the cluster turn-off, while the hotter component lies near an extension of the main sequence blueward of the turn-off, and thus qualifies as a blue straggler by itself. The reduction of the masses of the stars in the close binary compared to previous models indicates that it is possible that the more massive component of the close binary formed from a merger of just two turnoff-mass stars.(abridged)