The velocity dispersion profile of nine open clusters in the solar neighborhood

TL;DR

The paper addresses whether stellar-mass black holes influence the internal dynamics of nearby open clusters by inspecting velocity-dispersion profiles derived from Gaia DR3 data. It combines a two-Gaussian likelihood framework (to separate cluster members from field stars) with a binary-broadening model and uses MCMC to extract one-dimensional velocity dispersions for proper motions and radial velocities, subsequently building radial dispersion profiles. Direct N-body simulations (NBODY7) with realistic binaries, stellar evolution, and Galactocentric tides are used to generate theoretical dispersion profiles for comparison, enabling inferences about dynamical states and BH retention. The results suggest that NGC 2516 and NGC 3532 may host at least two stellar-mass BHs each and that some clusters are in an advanced dynamical state consistent with rapid evolution, though incomplete RV data and uncertainties in initial conditions limit definitive BH claims.

Abstract

We analyze the velocity dispersion profiles of nine open clusters in the solar neighborhood using kinematic data from Gaia DR 3, aiming to identify potential dynamical signatures of stellar-mass black holes through a comparison of theoretical and observed dispersion profiles. The selected clusters include LP2373 gp4, NGC 1980, NGC 2451A, NGC 2516, NGC 3532, NGC 6475, UBC 7, Praesepe, and Pleiades. We refine the center positions of the clusters with the Meanshift algorithm. Using the Markov Chain Monte Carlo method, we calculate the velocity dispersion for each cluster and construct one-dimensional velocity dispersion profiles. NGC 2516, NGC 3532, and NGC 6475 show potential central cusps in their radial velocity dispersion profiles, which may indicate the presence of stellar-mass black holes. LP2373 gp4, NGC 6475, and Praesepe all display a negative correlation between velocity dispersion and stellar mass, indicating these clusters are approaching energy equipartition or expanding. NGC 2516 and NGC 3532 exhibit a positive dependence between velocity dispersion and stellar mass, which may be attributed to the preferential ejection of massive stars following dynamical interactions involving binaries or black holes. These two clusters are the only two that are dynamical not relaxed and are closest to virial equilibrium. We compare the observations with N-body simulations of star clusters. A comparison of observed and simulated velocity dispersion profiles reveals that NGC 2516 and NGC 3532 exhibit lower proper motion dispersions than model clusters. Better agreement with the observed profiles is achieved for model clusters with larger ages. This suggests that the observed clusters may have undergone rapid dynamical evolution. Our results suggest that NGC 2516 and NGC 3532 may host at least two stellar-mass black holes each.

Figures (10)

• Figure 1: The $\mu_{\alpha}\cos\delta$ ($\mu_{\delta}$) and RV dispersion profiles of the four clusters: NGC 2516, NGC 3532, NGC 6475 and Pleiades. The red solid curve represents the 1D velocity dispersion profile. The black triangles represent the velocity dispersion of each subgroup. Distances from each subgroup to the cluster center are normalised using the half-mass radius $r_h$. The green bars are the errors in velocity dispersion for each subgroup. The grey dashed line represents the tidal radius of each cluster.
• Figure 2: The $\mu_{\alpha}\cos\delta$ ($\mu_{\delta}$) and RV dispersion profiles of the five clusters: LP 2373 gp4, NGC 1980, NGC 2451A, Praesepe and UBC 7. Colors and symbols are the same as in Fig. \ref{['fig:vdisp_prof_1']}.
• Figure 3: The $\mu_{\alpha}\cos\delta$ ($\mu_{\delta}$) and RV dispersion profiles with $r<2\,r_h$ of the four clusters: NGC 2516, NGC 3532, NGC 6475 and Pleiades. Colors and symbols are the same as in Fig. \ref{['fig:vdisp_prof_1']}.
• Figure 4: The $\mu_{\alpha}\cos\delta$ dispersion profiles of the nine clusters along stellar mass mass within tidal radius: LP2373 gp4, NGC 1980, NGC 2451A, NGC 2516, NGC 3532, NGC 6475, UBC 7, Praesepe, and Pleiades. The blue solid curve represents the 1D velocity dispersion profile. The black triangles represent the velocity dispersion of each subgroup. Masses from each subgroup are expressed in logarithmic scale. The orange bars represent the errors in the velocity dispersion for each subgroup.
• Figure 5: The $\mu_{\delta}$ dispersion profiles of the nine clusters along stellar mass mass within tidal radius: LP2373 gp4, NGC 1980, NGC 2451A, NGC 2516, NGC 3532, NGC 6475, UBC 7, Praesepe, and Pleiades. Colors and symbols are the same as in Fig. \ref{['fig:mass_sigma_ra']}.