Comprehensive Gaia DR3-Based Astrometric, Photometric, and Kinematic Studies of the Binary Open Cluster $h$ and $χ$ Persei

TL;DR

This work delivers a Gaia DR3-based, multi-dimensional assessment of the binary open cluster system NGC 869 (h Persei) and NGC 884 (χ Persei), combining membership via UP Mask, King-model structural fits, and Bayesian MCMC/SED methods to derive consistent ages, metallicities, extinctions, and distances. Kinematic and orbital analyses with galpy reveal similar space motions and circular Galactic orbits, indicating a common origin in the thin disk of the Perseus arm, while backward/forward orbital integration predicts a future close interaction at about 11 Myr and a minimum separation near 26 pc. The study also characterizes the mass distribution and dynamical state, finding near-Salpeter IMFs with mild mass segregation and relaxation times shorter than, but not far from, their ages, implying a mix of primordial and early dynamical effects. Overall, the results support a primordial, coeval binary open-cluster scenario with potential future tidal encounters, contributing important empirical constraints for cluster formation and evolution in the Milky Way.

Abstract

In the Gaia era, a comprehensive analysis of the binary open clusters NGC 869 (h Persei) and NGC 884 (chi Persei) system has been conducted to investigate its structural, astrophysical, kinematic, and Galactic orbital properties, along with its dynamical evolution. By applying the UPMASK algorithm to Gaia astrometric data for the estimation of cluster membership probabilities, it has been determined that 808 stars in NGC 869 and 707 stars in NGC 884 exhibit the highest statistical likelihood of being cluster members. The fundamental astrophysical parameters of the clusters were inferred within a Bayesian framework using Gaia data and PARSEC stellar evolutionary isochrones, through the application of the Markov Chain Monte Carlo (MCMC) technique. The estimated parameters are: colour excess E(B-V) = 0.516 +0.17/-0.24 mag and 0.516 +0.22/-0.33 mag, distances 2376 +301/-278 and 2273 +230/-290 pc, ages log(t/yr) = 7.31 +0.17/-0.32 and log(t/yr) = 7.30 +0.13/-0.29, and metallicities [Fe/H] = -0.24 +/- 0.12 and [Fe/H] = -0.25 +/- 0.12 dex for NGC 869 and NGC 884, respectively. Since spectroscopic observations are not available for the clusters, SED analysis was employed for the member stars, yielding results consistent with those obtained using the MCMC method. Kinematic and Galactic orbital analyses suggest that the open clusters originated in nearby regions of the Galaxy. This interpretation is supported by their similar space velocities and Galactic orbital parameters. Furthermore, orbital integration over 1 Gyr indicates a potential interaction between the clusters within the next 11 Myr. This study provides strong evidence of a common origin and a possible future dynamical interaction, contributing valuable insights into the formation and evolution of binary open clusters in the Milky Way.

Figures (17)

• Figure 1: Finder chart of the region containing the BOCs NGC 869 and NGC 884, obtained from the STScI Digitized Sky Survey (DSS). The red dashed circle indicates the area analyzed (left panel). Stellar surface density map of the same region, revealing two significant overdensities corresponding to NGC 869 (a) and NGC 884 (b) (right panel). Warmer colours represent higher stellar densities.
• Figure 2: The $G-$band star count histograms for NGC 869 and NGC 884 are presented, where the observational data are shown in black and the model predictions in red; the black dashed lines denote the photometric completeness limits.
• Figure 3: RDPs for NGC 869 (left panel) and NGC 884 (right panel) with best-fit King models (black lines). The magenta arrows indicate the limiting radii; the gray bands show background density levels. The $1\sigma$ fitting uncertainty is shown as a red-shaded band around the King profile.
• Figure 4: Distribution of membership probabilities assigned by the UPMASK algorithm for stars in the directions of the NGC 869 and NGC 884.
• Figure 5: Sky chart showing the spatial distribution of members for NGC 869 (black) and NGC 884 (red). The background image is taken from the STScI DSS.