The evolution of primordial binary open star clusters: Mergers, shredded secondaries, and separated twins

Abstract

The properties of the candidate binary star cluster population in the Magellanic Clouds and Milky Way are similar. The fraction of candidate binaries is ∼10% and the pair separation histogram exhibits a bimodal distribution commonly attributed to their transient nature. However, if primordial pairs cannot survive for long as recognizable bound systems, how are they ending up? Here, we use simulations to confirm that merging, extreme tidal distortion, and ionization are possible depending on the initial orbital elements and mass ratio of the cluster pair. Merging is observed for initially close pairs but also for wider systems in nearly parabolic orbits. Its characteristic timescale depends on the initial orbital semi-major axis, eccentricity, and cluster pair mass ratio, becoming shorter for closer, more eccentric equal mass pairs. Shredding of the less massive cluster and subsequent separation is observed in all pairs with appreciably different masses. Wide pairs evolve into separated twins characterized by the presence of tidal bridges and separations of 200-500 pc after one Galactic orbit. Most observed binary candidates appear to be following this evolutionary path which translates into the dominant peak (25-30 pc) in the observed pair separation distribution. The secondary peak at smaller separations (10-15 pc) can be explained as due to close pairs in almost circular orbits and/or undergoing merging. Merged clusters exhibit both peculiar radial density and velocity dispersion profiles shaped by synchronization and gravogyro instabilities. Simulations and observations show that long-term binary open cluster stability is unlikely. © 2010. The American Astronomical Society. All rights reserved.