The Evolution of Primordial Binary Open Star Clusters: Mergers, Shredded
  Secondaries and Separated Twins
  The basic properties of the candidate binary cluster population in the
Magellanic Clouds and Galaxy are similar. The fraction of candidate binary
systems is $\sim$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 pair. The nature of the dominant evolutionary
path largely depends on the strength of the local tidal field. Merging is
observed for initially close primordial binary clusters but also for wider
pairs 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 or
extreme tidal distortion of the less massive cluster and subsequent separation
is observed in all pairs with appreciably different masses. Wide pairs steadily
evolve into the separated twins state characterized by the presence of tidal
bridges and separations of 200-500 pc after one Galactic orbit. In the Galaxy,
the vast majority of observed binary candidates appear to be following this
evolutionary path which translates into the dominant peak (25-30 pc) in the
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. Both simulations and observations show that, for the range of
parameters studied here, long term binary cluster stability in the Galactic
disk is unlikely.
