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 ≳ 0.9. The ratio of the number of cluster binaries for which q > 0.75 to the number of binaries for which q > 0.5 (referred to as FQ75) ranges from ∼0.4 − 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 3-body dynamical processing of a primordial population of close binaries with initial mass ratios, q ≃ 1.