The Formation of Binary Star Clusters in the Milky Way and Large Magellanic Cloud

Abstract

Recent observations of young embedded clumpy clusters and statistical identifications of binary star clusters have provided new insights into the formation process and subsequent dynamical evolution of star clusters. The early dynamical evolution of clumpy stellar structures provides the conditions for the origin of binary star clusters. Here, we carry out 𝑁-body simulations in order to investigate the formation of binary star clusters in the Milky Way and in the Large Magellanic Cloud (LMC). We find that binary star clusters can form from stellar aggregates with a variety of initial conditions. For a given initial virial ratio, a higher degree of initial substructure results in a higher fraction of binary star clusters. The number of binary star clusters decreases over time due to merging or dissolution of the binary system. Typically, ∼ 45% of the aggregates evolve into binary/multiple clusters within 𝑡 = 20 Myr in the Milky Way environment, while merely ∼ 30% survives beyond 𝑡 = 50 Myr, with separations 50 pc. On the other hand, in the LMC, ∼ 90% of the binary/multiple clusters survive beyond 𝑡 = 20 Myr and the fraction decreases to ∼ 80% at 𝑡 = 50 Myr, with separations 35 pc. Multiple clusters are also rapidly formed for highly-substructured and expanding clusters. The additional components tend to detach and the remaining binary star cluster merges. The merging process can produce fast rotating star clusters with mostly flat rotation curves that speed up in the outskirts.