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

We use high-resolution Hubble Space Telescope imaging observations of the young (∼ 15 − 25 Myr) star cluster NGC 1818 in the Large Magellanic Cloud to derive an estimate for the binary fraction of F stars (1.3 < M⋆/M⊙ < 1.6). This study provides the strongest constraints yet on the binary fraction in a young star cluster in a low-metallicity environment ([Fe/H] ∼ −0.4). Employing artificial star tests, we develop a simplified method which can efficiently measure the probabilities of stellar blends and superpositions from the observed stellar catalogue. We create synthetic colour-magnitude diagrams matching the fundamental parameters of NGC 1818, with different binary fractions and mass ratio distributions. We find that this method is sensitive to binaries with mass ratios, q & 0.4. We find that, for binaries with F-star primaries and mass ratios q > 0.4, the binary fraction is ∼ 0.35. This suggests a total binary fraction for F stars of 0.6 to unity depending on assumptions about the form of the mass ratio distribution at low q.