The Frequency and Mass-Ratio Distribution of Binaries in Clusters -- III: Probabilistic Generative Modelling of Six Young Open Clusters

TL;DR

This study uses probabilistic generative models to analyze six young open star clusters, revealing trends in binary star frequency, mass-ratio distributions, and their evolution with cluster age.

Contribution

It introduces a probabilistic generative modeling approach to derive binary star properties and their dependence on cluster age and dynamics.

Findings

Younger clusters have higher binary incidence.

Mass-ratio distribution is mostly flat, with some clusters showing a sharp increase near q≈1.

Binary frequency ratio FQ_{75} increases with binary frequency but decreases with age.

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 $q\gtrsim0.9$. 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 $FQ_{75}$) ranges from $\sim0.4 - 0.8$. 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, $q \simeq 1$.