The frequency and mass-ratio distribution of binaries in clusters – III. Probabilistic generative modelling of six young open clusters

ABSTRACT 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, $q \simeq 1$.