Binary Fraction in Young Massive Star Clusters

Abstract

Binaries have strong impact on the evolution of stellar populations, they will accelerate the mass segregation process in a star cluster, form blue straggler stars. Some primordial scenario aim to explain the MSPs in globular clusters, have suggested massive binaries as the polluter of intra-cluster gas, and some evolutionary scenarios also invoke binaries as possible sources to produce the chemical anomalies in the stellar populations of globular clusters. All these scenarios are depend on the assumption that a cluster should initially contain a large fraction of binary stars, the evolutionary scenario also requires a large number of contacting binaries. In this Chapter, we take two young massive clusters, NGC 1805 and NGC 1818 as examples, to study their binary fractions and dynamics. We will see that the binary systems in these two clusters really occupy major fractions in number, and they were already experienced very complicated dynamical processes at this very early stage.