Revealing the Extended Stellar Structure of the Binary Cluster NGC 5617–Trumpler 22 with Gaia DR3

Abstract Binary clusters are important laboratories for studying star formation, the early dynamical evolution of open clusters, and interactions between stellar systems. NGC 5617–Trumpler 22 is a young binary cluster candidate in which the two clusters it contains have similar distances, ages, extinction values, and kinematic properties. The large-scale stellar structure of NGC 5617–Trumpler 22 remains insufficiently explored. As part of the Dali Binary Cluster Project (DL-BCP), this study aims to explore the large-scale stellar structure of the NGC 5617–Trumpler 22 binary cluster system and to search for external substructures associated with it. We used high-precision astrometric and photometric data from Gaia Data Release 3 (Gaia DR3) to analyse a region with a radius of 7.5○ centred on the system. Candidate members were selected by combining proper-motion, parallax, and colour–magnitude diagram (CMD) criteria, resulting in 4 111 candidate member stars. The spatial distribution of these members was further analysed using the density-based clustering algorithm DBSCAN. We identify an extended halo structure around NGC 5617–Trumpler 22, on a scale about six to eight times larger than the core radii of the two clusters. We also detect two previously uncatalogued external substructures at projected distances of about 55.61 and 152.94 pc from the binary cluster centre. These components show similar proper motions and parallaxes, and their CMDs indicate similar ages. Relative tangential motions suggest two possible origins: one may involve stars moving outwards from the central region, whereas the other may be a primordial remnant of the star-formation event that produced both clusters.