Twin Star Cities in Perseus: A Guide to the Gaia-Based Study of h & χ Persei

Taşdemir and collaborators present a comprehensive investigation of the famous “Double Cluster” in Perseus, NGC 869 (h Persei) and NGC 884 (χ Persei). These two open clusters are known for their close proximity on the sky and their similar stellar populations, making them excellent laboratories for studying how clusters form and evolve together. The authors use the high-precision measurements of the Gaia DR3 mission to explore the clusters’ structure, member stars, physical properties, and motion through the Milky Way. Their study aims to clarify whether these clusters share a common origin and how they may interact in the future.

Data and Initial Processing

The authors begin by outlining the scientific value of binary open clusters and reviewing why h and χ Persei have long been considered a true physical pair. They then describe the Gaia DR3 data used, including stellar positions, motions across the sky (proper motions), brightnesses, and parallaxes (a measure of distance). After applying quality filters to remove unreliable measurements, they examine nearly 92,000 stars in the region. Using star counts as a function of brightness, they identify a “completeness limit” at magnitude G = 20.5, beyond which Gaia does not reliably detect all stars. This ensures that only high-quality data contribute to their structural and astrophysical analyses.

Cluster Structure and Membership

Next, the team studies the clusters’ structures by examining how the density of stars changes with radius from each cluster’s center. They fit these trends with a King model, a commonly used formula that describes how star density decreases from a dense central core to a more diffuse outer region. They find that both clusters have limiting radii of about 19 arcminutes and moderate central concentrations, suggesting that they are young systems still in the early stages of dynamical evolution. To identify which stars truly belong to each cluster, the authors apply the UPMASK algorithm, which groups stars using their positions, colors, and motions. This process identifies 808 member stars for NGC 869 and 707 for NGC 884, with negligible overlap between the two groups.

Astrophysical Properties via MCMC and SED Fitting

With the member stars established, the authors determine each cluster’s physical properties. They first analyze color–magnitude diagrams, plots that show how bright stars are compared to their colors, to derive ages, distances, metallicities, and reddening (dust absorption). Using a Bayesian Markov Chain Monte Carlo (MCMC) method, they fit theoretical PARSEC stellar models to the observations. They find both clusters to be about 20 million years old (log(t/yr) ≈ 7.30), slightly metal-poor ([Fe/H] ≈ –0.25), and located around 2.3–2.4 kiloparsecs from Earth. They further confirm these results with a second, independent method: spectral energy distribution (SED) fitting, which compares each star’s brightness across many wavelengths to model predictions. The SED-based distances, reddening values, and ages agree closely with the MCMC results, strengthening the reliability of the derived properties.

Kinematics and Galactic Orbit Modeling

Finally, the authors examine how the clusters move within the Milky Way by combining Gaia proper motions, radial velocities from Gaia and large spectroscopic surveys, and their derived distances. Using the galpy orbital modeling software, they compute the three-dimensional velocities and Galactic orbits of both clusters. The two clusters exhibit nearly identical space motions and orbit the Galaxy along similar paths. Their orbital simulations indicate that h and χ Persei were born in nearby regions of the Galaxy and may even come close enough to interact dynamically within the next 11 million years. This supports the idea that they formed together, likely from the same giant molecular cloud, and have remained a bound or semi-bound pair ever since.

Conclusions

Overall, the paper provides strong evidence that NGC 869 and NGC 884 constitute a genuine binary open cluster system. Through careful analysis of Gaia DR3 data, supported by advanced statistical methods and orbital modeling, the authors show that these two clusters share the same age, similar structure, almost identical chemical composition, and coherent Galactic motion. Their findings contribute important insights into how binary clusters form, evolve, and interact within the broader environment of the Milky Way.

Source: Taşdemir