Hubble Space Telescope survey of Magellanic Cloud star clusters. Binaries among the split main sequences of NGC 1818, NGC 1850, and NGC 2164

Nearly all star clusters younger than ∼600 Myr exhibit extended main sequence turnoffs and split main sequences (MSs) in their color-magnitude diagrams. Works based on both photometry and spectroscopy have clearly demonstrated that the red MS is composed of fast-rotating stars, whereas blue-MS stars are slow rotators. Nevertheless, the mechanism responsible for the formation of stellar populations with varying rotation rates remains a topic of debate. Potential mechanisms proposed for the split MS include binary interactions, the early evolution of pre-MS stars, and the merging of binary systems, but a general consensus has yet to be reached. These formation scenarios predict different fractions of binaries among blue- and red-MS stars. Therefore, studying the binary populations can provide valuable constraints that may help clarify the origins of the split MSs. We used high-precision photometry from the Hubble Space Telescope to study the binaries of three young Magellanic star clusters exhibiting split MSs, namely NGC 1818, NGC 1850, and NGC 2164. By analyzing the photometry in the F225W, F275W, F336W, and F814W filters for observed binaries and comparing it to a large sample of simulated binaries, we determined the fractions of binaries within the red and blue MS. We find that the fractions of binaries among the blue-MS stars are higher than those of red-MS stars by a factor of ∼1.5, 4.6, and ∼1.9 for NGC 1818, NGC 1850, and NGC 2164, respectively. We discuss these results in the context of the formation scenarios of the split MS.