Internal dynamics and structure of Cepheus OB4. The asymmetric expansion of Berkeley 59

Abstract

Accurate measurements of young stellar cluster internal dynamics provide crucial insights into their formation. With Gaia, we are now able to trace stellar motions and study the dynamics of star clusters with unprecedented precision, but this requires a reliable list of probable members. We examine a 2 deg-radius region in Cepheus OB4, centered on the young cluster Berkeley 59, to build a reliable candidate member list, enabling the study of the cluster's structure, kinematics, and stellar population. We compiled a catalog of optical and near-infrared photometry, along with precise positions and proper motions from Gaia DR3, for sources in the Cepheus OB4 field. Membership probabilities were determined using a probabilistic random forest algorithm and further refined by requiring HR diagram positions consistent with a young age. From a list of 1030 probable members, we estimate a distance of 1009+-12 pc to Berkeley 59. Masses, extinction, and ages were derived by fitting the spectral energy distributions to atmospheric and evolutionary models, while internal dynamics was analyzed using proper motions relative to the cluster's mean motion. Berkeley 59 exhibits an asymmetric expansion pattern with velocity increasing outward and a preferred motion toward the north. The IMF between 0.4 and 7 MSun follows a single power law (dN/dM \propto M**-alpha), with the slope alpha=2.3+-0.3, consistent with Salpeter's slope and previous studies in the region. The region's median age, estimated from the HR diagram, is 2.9 Myr. The velocity dispersion of Berkeley 59 exceeds the virial velocity dispersion derived from its total mass (650+-30 MSun) and half-mass radius (1.71+-0.13 pc). The 2D motions of a stellar group located about 1 deg north of Berkeley 59 provide further support for the previously proposed triggered star formation scenario. (Abridged)

Figures (19)

• Figure 1: Density of sources in our final catalog as a function of magnitude.
• Figure 2: Gaia DR3 proper motions of objects in Cepheus OB4 (grey dots). Black dots represent the 860 candidate members from the literature within a $30\arcmin$ radius of Berkeley 59 center. The orange ellipse indicates the proper motion selection criterion for constructing the member training set. In the lower right corner, we show the mean proper motion uncertainty.
• Figure 3: Color-magnitude diagrams used for training set selection. Black dots in the top panels are candidate members from the literature, while the grey dots mark all the sources in the field. The dashed lines show the selection criteria, and the solid lines represent the 2 Myr PARSEC isochrones, shifted to a distance of 1100 pc Kuhn:berk59_data_young_clusters_gaia. In the lower panels, the black dots represent the probable members selected for the training set, after the proper motion, color, and parallax filters.
• Figure 4: Parallax measurements for the member training set candidates, selected based on proper motions and colors (black dots). The dotted orange line represents the weighted average of the parallaxes, while the shaded area marks $\pm1\sigma$ range. The red crosses mark the sources that have been excluded from the final member training set (see text for the details of the applied criterion).
• Figure 5: Distribution of the membership probability of the final catalog limited to objects $30 \arcmin$ from the center for clarity. The four leftmost columns are capped for better readability. The inset diagram shows the prediction for all objects labelled as members in the training set. The rightmost column of the inset diagram is capped and its value depicted next to it.