The origin of very wide binary stars
  A large population of fragile, wide (> 1000 AU) binary systems exists in the
Galactic field and halo. These wide binary stars cannot be primordial because
of the high stellar density in star forming regions, while formation by capture
in the Galactic field is highly improbable. We propose that these binary
systems were formed during the dissolution phase of star clusters (see
Kouwenhoven et al. 2010, for details). Stars escaping from a dissolving star
cluster can have very similar velocities, which can lead to the formation of a
wide binary systems. We carry out N-body simulations to test this hypothesis.
The results indicate that this mechanism explains the origin of wide binary
systems in the Galaxy. The resulting wide binary fraction and semi-major axis
distribution depend on the initial conditions of the dissolving star cluster,
while the distributions in eccentricity and mass ratio are universal. Finally,
since most stars are formed in (relatively tight) primordial binaries, we
predict that a large fraction of the wide "binary stars" are in fact
higher-order multiple systems.
