Consequences of Dynamical Disruption and Mass Segregation for the Binary
  Frequencies of Star Clusters
  The massive (13,000-26,000 Msolar), young (15-30 Myr) Large Magellanic Cloud
star cluster NGC 1818 reveals an unexpected increasing binary frequency with
radius for F-type stars (1.3-2.2 Msolar). This is in contrast to many older
star clusters that show a decreasing binary frequency with radius. We study
this phenomenon with sophisticated N-body modeling, exploring a range of
initial conditions, from smooth virialized density distributions to highly
substructured and collapsing configurations. We find that many of these models
can reproduce the cluster's observed properties, although with a modest
preference for substructured initial conditions. Our models produce the
observed radial trend in binary frequency through disruption of soft binaries
(with semi-major axes, a > 3000 AU), on approximately a crossing time (~ 5.4
Myr), preferentially in the cluster core. Mass segregation subsequently causes
the binaries to sink towards the core. After roughly one initial half-mass
relaxation time (t_rh(0) ~ 340 Myr) the radial binary frequency distribution
becomes bimodal, the innermost binaries having already segregated towards the
core, leaving a minimum in the radial binary frequency distribution that
marches outwards with time. After 4-6 t_rh(0), the rising distribution in the
halo disappears, leaving a radial distribution that rises only towards the
core. Thus, both a radial binary frequency distribution that falls towards the
core (as observed for NGC 1818) and one that rises towards the core (as for
older star clusters) can arise naturally from the same evolutionary sequence
owing to binary disruption and mass segregation in rich star clusters.
