Stars usually form in clusters, which can also form in pairs or groups. Binary clusters (BCs) are defined as pairs of open clusters closely associated both in position and kinematics. They provide insight into how stars form within giant molecular clouds, making them important indicators of star formation and cluster evolution.

Using high-precision astrometry from the Gaia satellite and applying uniform, stringent selection criteria, researchers from the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences (CAS) have identified 400 candidate binary open clusters in the Milky Way; 268 of these were newly reported. The findings, published in Astronomy & Astrophysics, provide a unified, structured scheme for identifying and classifying galactic binary clusters. 

This work was conducted by PhD candidate LIU Guimei and her supervisor Prof. ZHANG Yu from XAO, together with their collaborators from the Shanghai Astronomical Observatory of CAS. 

They analyzed nearly 4,000 high-quality open clusters using Gaia DR3 astrometry and kinematics. They established a statistical, quantitative criterion for spatial and velocity proximity and validated it against randomized mock samples. Using this framework, the researchers identified 400 BC candidates and classified them into three categories: (i) primordial binary clusters (co-natal), (ii) tidal-capture/resonant-capture binary clusters, and (iii) optical pairs (chance alignments).

Further analysis shows that 61% of the candidate binary clusters are highly consistent in age and kinematics, supporting formation from the same giant molecular cloud, and 83% display significant tidal interactions. The strength of the interaction correlates clearly with spatial separation—the closer the pair, the stronger the mutual attraction and perturbation.

Overall, about 17% of open clusters are currently in binary or multiple-cluster systems, and roughly 10% likely formed as primordial binary clusters. These fractions align well with previous theoretical and observational estimates. 

Cross-matching with previously reported BCs shows that the method recovers a large fraction of known systems. Despite stricter selection criteria, it also adds 268 newly identified physical BCs to the galactic sample.

This study suggests that hierarchical star formation is an important process and provides key observational evidence for the formation mechanisms and dynamical evolution of multi-cluster systems. This evidence supports a hierarchical, clustered scenario of star formation across multiple scales.

This work received positive feedback during peer review. One anonymous referee noting: "This is a good paper that offers new insights into a topical issue."

This study was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences, and Xinjiang regional funds.