2025MNRAS.536..471A

We apply probabilistic generative modelling of colour-magnitude diagrams to six young Galactic open star clusters and determine their mass functions, binary mass-ratio distributions, and the frequencies of binary stars. We find that younger clusters tend to exhibit a higher incidence of binaries than their older counterparts. The mass-ratio distribution is fairly flat for the clusters, with one exception that exhibits a sharp increase for $q\gtrsim 0.9$. The ratio of the number of cluster binaries for which $q\gt 0.75$ to the number of binaries for which $q\gt 0.5$ (referred to as $FQ_{75}$) ranges from $\sim 0.4 \,{\text{to}}\, 0.8$. This metric increases with the binary-star frequency of a cluster but declines with cluster age. This may be due to non-ionizing three-body dynamical processing of a primordial population of close binaries with initial mass ratios, $q \simeq 1$.