Binary clusters in the Galactic disk I: Systematic identification and classification using Gaia DR3

TL;DR

This work identifies and classifies binary and multiple open clusters (BCs) in the Galactic disk using Gaia DR3–based high-precision 6D astrometry and isochrone ages. By constructing a homogeneous sample of $N=4{,}084$ clusters from the HR24 catalog and calibrating proximity criteria with mock data, the authors identify $400$ BC candidates within $\Delta D_{3D} \leq 50$ pc and $\Delta V_t \leq 10$ km s$^{-1}$, labeling them Golden or Full by radial-velocity availability. They classify BCs into Primordial Binary Clusters (PBC, $\Delta V_{3D} \leq 20$ km s$^{-1}$, $\Delta\mathrm{Age} \leq 30$ Myr), Tidal Capture/Resonant Trapping BCs (TBC, $\Delta V_{3D} \leq 20$ km s$^{-1}$, $\Delta\mathrm{Age} > 30$ Myr), or Hyperbolic Encounter Pairs (HEP, $\Delta V_{3D} > 20$ km s$^{-1}$). They further compute a tidal factor TF = $d^{3}/(M_{cl}R_{50})$ and identify 82 OC groups comprising 278 clusters, including 27 new groups, revealing a high prevalence of tidal interactions and a strong presence of primordial BCs. Cross-matching with literature confirms many known systems and adds 268 new BCs, indicating that about $16.8\%$ of OCs participate in BCs or groups and $\sim 9.9\%$ are likely born in primordial BCs. Overall, the study provides a comprehensive, homogeneous catalog of Galactic BCs and robust observational constraints on the formation and dynamical evolution of multiple cluster systems.

Abstract

Aims. We aim to identify and classify BCs using high-precision astrometric and kinematic data, and to investigate their physical properties, mutual gravitational interactions, and formation rates. Methods. We used a comprehensive star cluster catalog that contains 4,084 high-quality clusters. Based on spatial and kinematic proximity, we identified 400 cluster pairs involving 686 unique clusters. These pairs were classified into three types: primordial BCs, systems formed through tidal capture or resonant trapping, and hyperbolic encounter pairs. For each system, we calculated the tidal factor to quantify the strength of mutual tidal interaction. Additionally, we constructed multi-cluster systems by identifying transitive connections among cluster pairs. Results. Among the 400 identified cluster pairs, nearly 60.8% (243 pairs) are probably primordial BCs, exhibiting both similar ages and motions. This supports a scenario where they formed together in the same giant molecular cloud. We find that 82.5% of the cluster pairs have strong mutual tidal forces. In addition, 278 star clusters are identified as members of 82 multi-cluster systems, including 27 newly reported groups. Cross-matching with the literature confirms the recovery of previously reported systems and leads to the discovery of 268 new cluster pairs. In our sample, about 16.8% of star clusters are involved in some type of interaction with another cluster, and 9.94% of star clusters are likely born in primordial BCs. Conclusions. Our results provide a comprehensive, homogeneously identified sample of Galactic BCs. The high fraction of primordial BCs and their mutual tidal interaction suggest that cluster formation in pairs is a main outcome of star formation. This work offers new observational constraints on the formation and dynamical evolution of multiple star cluster systems.

Figures (8)

• Figure 1: Comparison of the distributions for the spatial separation and velocity difference of the nearest neighbours. The salmon histogram is the result of one mock trial, while the blue histograms correspond to the results from our working sample.
• Figure 2: Distribution of peak values of spatial separation and velocity difference for nearest neighbours in 1000 mock trials.
• Figure 3: Distribution of different types of cluster pairs and sample category. The blue bars represent the total number of pairs in each type of cluster pairs, the orange bars indicate those in the full sample, and the red bars correspond to the golden sample. The numbers above each bar indicate the total number of pairs in that subcategory.Cluster pair types include primordial binary clusters (PBCs), tidal capture or resonant trapping binary clusters (TBCs), and Hyperbolic encounter pairs (HEPs), as defined in Section \ref{['sec: classfy']}
• Figure 4: Multi-dimensional distributions of representative cluster pairs of different types. Shown are the distributions of member stars in spatial coordinates (left panel), radial velocity (middle panel), and color-magnitude diagram (CMD; right panel). Each point is colored according to the OC to which the star belongs. Arrows indicate the tangential velocities of the clusters, with each cluster pair plotted using a distinct velocity scale for clarity.
• Figure 5: Mass ratio versus spatial separation for pairs with $\Delta D_{3D} \leq$ 50. Gray symbols represent pairs from the initial sample, while blue symbols represent cluster pairs identified in this work. Circles indicate pairs with TF values less than 200, and triangles indicate those with TF values greater than 200.