Tracing the Galactic Disk with Gaia DR3: A Deep Study of Berkeley 17, 18, and 39 Open Star Clusters

TL;DR

This work exploits Gaia DR3 to perform a comprehensive, membership-driven analysis of Berkeley 17, 18, and 39, deriving distances, ages, reddening, structural parameters, and mass functions from decontaminated CMDs and King-model fits. By integrating isochrone fitting with precise astrometry and kinematics, the authors establish dynamical ages via relaxation times, reveal near-Salpeter-like mass functions, and reconstruct the clusters’ orbits with Galpy in MW potentials, showing Be17 and Be39 as dynamically evolved outer-disk members with Be18 on a nearly circular outer-disk path. The results illuminate the outer Galactic disk’s formation history, chemical evolution, and dynamical processes, highlighting the value of old open clusters as chronometers and tracers of Galactic structure. Overall, the study provides refined fundamental parameters and a cohesive picture of the three clusters’ structure, kinematics, and Galactic context, reinforcing their roles as probes of the thick-disk/outward disk population.

Abstract

We report a detailed investigation of three intermediate-to-old age open clusters, Berkeley 17, Berkeley 18, and Berkeley 39, utilizing precise astrometric and photometric data from Gaia DR3. Cluster membership was robustly determined through a probabilistic proper-motion analysis, yielding statistically significant samples of 600, 1042, and 907 stars, respectively. From the mean parallaxes of these members, we determine astrometric distances ranging from approximately 3.40 kpc for Berkeley 17 to 5.80 kpc for Berkeley 18. Isochrone fitting applied to the decontaminated color-magnitude diagrams constrains the cluster ages to 9.12 +/- 1.00 Gyr, 3.36 +/- 0.50 Gyr, and 5.10 +/- 0.50 Gyr, respectively. Interstellar reddening spans a wide range, from E(B-V) = 0.17 mag in Berkeley 39 to 0.58 mag in Berkeley 17. Structural parameters derived from King model fits to the radial density profiles, combined with mass function analyses, indicate that the clusters are dynamically relaxed systems with mass distributions broadly consistent with the canonical Salpeter slope. Our kinematic analysis reveals that Berkeley 17, Berkeley 18, and Berkeley 39 are part of the outer disk population.

Figures (11)

• Figure 1: DSS images illustrating the locations of the open clusters Be17, Be18, and Be39 within the MW.
• Figure 2: Completeness histograms for the open clusters Be17, Be18, and Be39. The red dashed line in each panel indicates the estimated photometric completeness limit in the $Gaia~G$-apparent magnitude.
• Figure 3: The two-dimensional Gaussian modeling was employed to determine the central coordinates of the stellar density enhancements in $\alpha$ and $\delta$. The resulting density maps and contour distributions reveal the spatial structure and stellar concentration patterns of the Berkeley OCs investigated in this study. Pink dashed lines indicate the center of the clusters.
• Figure 4: The RDPs of the investigated OCs represented by the blue solid line. The dashed red line represents the King1962 fitting while the red solid line and blue dashed line the background density $\rho_{\rm bg}$ and the uncertainties of the background density, respectively.
• Figure 5: Normalized trigonometric parallax distribution of stars with membership probability $P \geq 0.5$ for Be17, Be18, and Be39. The red curve represents the best-fit Gaussian to the distribution.