Dynamical detection of three triple stellar systems in open clusters

Abstract

We present a kinematic analysis of three triple stellar systems belonging to two open clusters: CPD−60°961 and HD 66137 in NGC 2516, and HD 315031 in NGC 6530. All three systems are hierarchical triples with a close binary bound to a third body in a wider orbit, whose presence is detected through velocity variations of the close binary barycentre. Orbital parameters are derived from radial velocity curves. Absolute parameters for all stars are estimated assuming cluster membership. Some dynamical and evolutionary aspects of these systems are discussed, particularly the possible influence of Kozai cycles. The two systems of NGC 2516 have similar orbital configurations with inner periods of 11.23 and 8.70 d and outer periods of 9.79 and 9.24 yr. The young system HD 315031 in the cluster NGC 6530 has an inner binary with a period of 1.37 d and a very eccentric (e = 0.85) outer orbit with a period of 483 d. We report also radial velocity measurements of the components of the visual binary CPD−60°944 in NGC 2516. Including results from previous works, this cluster would harbour five hierarchical triples. Possible dynamical evolutionary scenarios are discussed. Long-term radial velocity monitoring is highlighted as strategy for the detection of subsystems with intermediate separations, which are hard to cover with normal spectroscopic studies or visual techniques.