The binary star population of the young cluster NGC 1818 in the Large Magellanic Cloud

Abstract

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 M_primary~2-5.5M_solar and M_secondary>~0.7M_primary. 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.