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 (similar to U) and F555W (similar to V). Our sample includes binaries with M(primary)similar to 2-5.5 M-. and M-secondary greater than or equal to 0.7M(primary). The binary fraction increases towards the cluster centre, Rom similar to 20 +/- 5 per cent in the outer parts, to similar 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.
Monthly Notices of the Royal Astronomical Society,
Vol. 300, no. 3 (November 1998), p. 857-862