We determine the binary star fraction as a function of radius in NGC 1818, a young rich cluster in the Large Magellanic Cloud, using HST images in F336W ($\sim U$) and F555W ($\sim V$). Our sample includes binaries with $M_{primary} \sim 2 - 5.5 \msun$ and $M_{secondary} \gtsim 0.7 M_{primary}$. The binary fraction increases towards the cluster centre, from $\sim 20$\% in the outer parts, to $\sim 35$\% 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.
Back to top of the abstracts page.