Binary Mass Fraction in Open Clusters

Most stars in the Universe evolve in binary systems, where two gravitationally bound stars orbit around a mutual center of mass. Studying this type of system is essential for our understanding of star formation and evolution, as well as tracing the history of our galaxy. In this research, we investigate the binary mass fraction distribution of various open clusters in the Milky Way. By data mining Gaia DR3 astrometry— a rich catalogue of the Milky Way— we collect information such as position, proper motion, and parallax of the stars situated near the clusters. We determine the probability of cluster membership for each star, be it single or double, using Maximum Likelihood Estimation. Then we model the observed binary mass ratio distribution with a power law. We estimate the binary fraction and the power law exponent from the cluster Color-Magnitude diagrams. We find that, in every cluster, equal mass binary fraction is more likely to occur than unequal mass. This observation constrains theories of star and planet formation.