Tidal tails in open clusters: Morphology, binary fraction, dynamics, and rotation

TL;DR

This work addresses the detection and characterization of tidal tails around open clusters using Gaia DR3 data and unsupervised learning. By combining DBSCAN and PCA to identify core members and elongated structures, RV and CMD filtering, tangential projection, and N-body-inspired references, the authors map tails beyond the Jacobi radius $r_J$ for five clusters (BH 164, Alessi 2, NGC 2281, NGC 2354, M67). They further analyze tail/internal dynamics, luminosity functions, and binary fractions, and reveal rotation signatures in M67 and NGC 2281, with tails often showing higher binary fractions and a deficit of high-mass stars. The results extend tail detection to more distant systems, validate tidal-origin morphology with orbit-based orientation, and provide observational constraints for cluster dissolution and Galactic potential models. The approach demonstrates the value of combining ML techniques with dynamical analysis to uncover hidden tidal structures and their kinematic signatures in the Galactic disk.

Abstract

Context: This research presents unsupervised machine learning and statistical methods to identify and analyze tidal tails in open star clusters using data from the Gaia DR3 catalog. Aims: We aim to identify member stars and to detect and analyze tidal tails in five open clusters, BH 164, Alessi 2, NGC 2281, NGC 2354, and M67, of ages between 60 Myr and 4 Gyr. These clusters were selected based on the previous evidence of extended tidal structures. Methods: We utilized machine learning algorithms such as Density-based Spatial Clustering of Applications with Noise (DBSCAN) and principal component analysis (PCA), along with statistical methods, to analyze the kinematic, photometric, and astrometric properties of stars. Key characteristics of tidal tails, including radial velocity, the color-magnitude diagram, and spatial projections in the tangent plane beyond the cluster's Jacobi radius, were used to detect them. We used N-body simulations to visualize and compare the observables with real data. Further analysis was done on the detected cluster and tail stars to study their internal dynamics and populations, including the binary fraction. We also applied the residual velocity method to detect rotational patterns in the clusters and their tails. Results: We identified tidal tails in all five clusters, with detected tails extending farther in some clusters and containing significantly more stars than previously reported (tails ranging from 40 to 100 pc, one to four times their Jacobi radius, with 100-200 tail stars). The luminosity functions of the tails and their parent clusters were generally consistent, and tails lacked massive stars. In general, the binary fraction was found to be higher in the tidal tails. Significant rotation was detected in M67 and NGC 2281 for the first time.

Figures (12)

• Figure 1: Spatial distribution, proper motion, and parallax of the filtered sample (light coral) after filtering and the cluster members (blue) after applying DBSCAN for M67.
• Figure 2: Spatial distribution of the identified core members (blue dots) for the five clusters with their detected tidal tails (red and coral dots), N-body simulated cluster (empty dots), Jacobi limit (dotted black circle), and expected orbit (gray arrow) pointing toward the direction of motion (refer to Subsect. \ref{['subsec:orbit']}). Crosses indicate manually removed stars for NGC 2281.
• Figure 3: First-order corrected proper motion vectors for the five open clusters.
• Figure 4: Mean residual velocities shown as a function of the three position angles ($\alpha$, $\beta$, $\gamma$) for the two subsamples in the top panel for M67 within 5$r_J$. Difference ($\Delta$) in the mean residual velocities ($v_{x_c}$, $v_{y_c}$, and $v_{z_c}$) of the two subsamples is displayed as a function of the position angles (PA) in the bottom panel. Error bars (in gray), best-fitting sine functions (in red), and the initially fitted angles are also shown.
• Figure 5: Rotational velocity component as a function of the distance $r$ from the cluster center for the cluster + tail stars within 120 pc of M67. Similar plot for NGC 2281 in Fig. \ref{['fig:A6']}.