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 old) 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 dex). Employing artificial-star tests, we develop a simple method that can efficiently measure the probabilities of stellar blends and superpositions from the observed stellar catalog. We create synthetic color-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. 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 0.55 to unity, depending on assumptions about the form of the mass-ratio distribution at low q.