We determine the binary star fraction as a function of radius in NGC 1818, a young rich cluster in the Large Magellanic Cloud, using Hubble Space Telescope images in bands F336W (∼U) and F555W (∼V). Our sample includes binaries with Mprimary ∼ 2–5.5 M⊙ and Msecondary ≳ 0.7Mprimary. The binary fraction increases towards the cluster centre, from ∼20 ± 5 per cent in the outer parts, to ∼35 ± 5 per cent inside the core. This increase is consistent with dynamical mass segregation and need not be primordial. We compare our results with expectations from N-body models, and discuss the implications for the formation and early evolution of such clusters.
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Elson et al. (1998) studied this question.