The Hierarchical Structure of the Open Cluster NGC 752

TL;DR

This study tackles how an old open cluster, NGC 752, is internally structured and dissolving under Galactic tides. It applies a four-dimensional Gaia DR3–based hierarchical clustering method, using a binary-tree constructed from projected binding energy to identify four substructures and characterize their kinematics. The results reveal pronounced mass segregation, a radial expansion of outer members at $0.25\,\mathrm{km\,s^{-1}}$, and a residual rotation of $0.029\,\mathrm{rad\,Myr^{-1}}$, with escape histories linked to successive Galactic disk crossings. The work highlights the utility and limits of projection-based hierarchical clustering for internal cluster structure and underscores the need for complementary techniques to trace escaped members and the full dynamical history.

Abstract

The structure of open clusters provides key insights into their evolution and the dynamics of the Milky Way. Using Gaia DR3 data, we applied a hierarchical clustering algorithm to the open cluster NGC 752 based on the kinematical information and identified four substructures corresponding to different stages of disintegration. The cluster exhibits a pronounced signature of mass segregation. Its outer members show a clear expansion trend with a velocities of 0.25 $\rm{km~s^{-1}}$ in the plane of the sky. In addition, the system shows a projected rotational pattern with an angular velocity of approximately 0.03 $\rm{rad~Myr^{-1}}$. We also identified a correlation between the escape times of disturbed members and the epochs at which the cluster crossed the Galactic disk, highlighting the role of Galactic tidal forces in accelerating cluster dissolution. We conclude that hierarchical clustering based on projection bounding energy is effective for studying the internal structure of star clusters, but it has limitations when dealing with unconstrained structures such as tidal tails.

Figures (13)

• Figure 1: The $w_0$-$p$ plot. $w_0$ represents the proportion of the largest plateau when the algorithm performs Gaussian fitting on different plateaus.
• Figure 2: Left: Velocity dispersion profile, with leaf nodes on the x-axis and velocity dispersion values on the y-axis. The gray line corresponds to largest velocity plateau 1.02 $\rm{km~s^{-1}}$. Other four colored lines, from low to high, representing 0.788, 0.844, 0.916, and 0.977 $\rm{km~s^{-1}}$, represent the thresholds that separate different hierarchical substructures. Right: Histogram of velocity dispersion, with solid colored lines representing multiple Gaussian components.
• Figure 3: The member of the star cluster NGC 752 selected by H23 (solid dots) and this work (red blank triangles). The probabilities of H23 member stars are marked with colors.
• Figure 4: The dendrogram of NGC 752, with the y-axis representing velocity dispersion. Blue indicates the Sub 1, orange Sub 2, green Sub 3, and pink Sub 4.
• Figure 5: The spatial distribution of four substructures of NGC 752. Blue circles represent the Sub 1, orange squares represent the Sub 2, green inverted triangles represent the Sub 3, and pink triangles represent the Sub 4. The center of the cluster is indicated by a black plus sign, and the tidal radius(9.5 pc) is indicated by a gray circle.