The massive ( 13,000–26,000 M _ { \odot } ) , 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 M _ { \odot } ) .
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 \gtrsim 3000 AU ) , on approximately a crossing time ( \sim 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 ) \sim 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 .