The binary fractions in the massive young Large Magellanic Cloud star
  clusters NGC 1805 and NGC 1818
  Using high-resolution data sets obtained with the Hubble Space Telescope, we
investigate the radial distributions of the F-type main-sequence binary
fractions in the massive young Large Magellanic Cloud star clusters NGC 1805
and NGC 1818. We apply both an isochrone-fitting approach and chi^2
minimization using Monte Carlo simulations, for different mass-ratio cut-offs,
q, and present a detailed comparison of the methods' performance. Both methods
yield the same radial binary fraction profile for the same cluster, which
therefore supports the robustness and applicability of either method to young
star clusters which are as yet unaffected by the presence of multiple stellar
populations. The binary fractions in these two clusters are characterized by
opposite trends in their radial profiles. NGC 1805 exhibits a decreasing trend
with increasing radius in the central region, followed by a slow increase to
the field's binary-fraction level, while NGC 1818 shows a monotonically
increasing trend. This may indicate dominance of a more complicated physical
mechanism in the cluster's central region than expected a priori. Time-scale
arguments imply that early dynamical mass segregation should be very efficient
and, hence, likely dominates the dynamical processes in the core of NGC 1805.
Meanwhile, in NGC 1818 the behavior in the core is probably dominated by
disruption of soft binary systems. We speculate that this may be owing to the
higher velocity dispersion in the NGC 1818 core, which creates an environment
in which the efficiency of binary disruption is high compared with that in the
NGC 1805 core.
