THE EVOLUTION OF PRIMORDIAL BINARY OPEN STAR CLUSTERS: MERGERS, SHREDDED SECONDARIES, AND SEPARATED TWINS

Abstract

The basic properties of the candidate binary cluster population in the\nMagellanic Clouds and Galaxy are similar. The fraction of candidate binary\nsystems is $\\sim$10% and the pair separation histogram exhibits a bimodal\ndistribution commonly attributed to their transient nature. However, if\nprimordial pairs cannot survive for long as recognizable bound systems, how are\nthey ending up? Here, we use simulations to confirm that merging, extreme tidal\ndistortion and ionization are possible depending on the initial orbital\nelements and mass ratio of the pair. The nature of the dominant evolutionary\npath largely depends on the strength of the local tidal field. Merging is\nobserved for initially close primordial binary clusters but also for wider\npairs in nearly parabolic orbits. Its characteristic timescale depends on the\ninitial orbital semi-major axis, eccentricity, and cluster pair mass ratio,\nbecoming shorter for closer, more eccentric equal mass pairs. Shredding or\nextreme tidal distortion of the less massive cluster and subsequent separation\nis observed in all pairs with appreciably different masses. Wide pairs steadily\nevolve into the separated twins state characterized by the presence of tidal\nbridges and separations of 200-500 pc after one Galactic orbit. In the Galaxy,\nthe vast majority of observed binary candidates appear to be following this\nevolutionary path which translates into the dominant peak (25-30 pc) in the\npair separation distribution. The secondary peak at smaller separations (10-15\npc) can be explained as due to close pairs in almost circular orbits and/or\nundergoing merging. Merged clusters exhibit both peculiar radial density and\nvelocity dispersion profiles shaped by synchronization and gravogyro\ninstabilities. Both simulations and observations show that, for the range of\nparameters studied here, long term binary cluster stability in the Galactic\ndisk is unlikely.\n