Internal dynamics and structure of Cepheus OB4

The asymmetric expansion of Berkeley 59

¹ Fakultät für Physik und Erdsystemwissenschaften, Universität Leipzig, Linnéstraße 5, 04103 Leipzig, Germany
² Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8, Campo Grande, 1749-016 Lisbon, Portugal
³ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisbon, Portugal
⁴ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

^★ Corresponding author: kmuzic@fc.ul.pt

Received: 1 April 2025
Accepted: 18 September 2025

Abstract

Context. Accurate measurements of the internal dynamics of young stellar clusters can be used to extract crucial information about their formation process. With Gaia, we are now able to trace stellar motions and study the dynamics of star clusters with unprecedented precision. A fundamental requirement for this analysis is a well-defined and reliable list of probable members.

Aims. We examined a region with a radius of 2° in Cepheus OB4, centered on the young cluster Berkeley 59, to build a reliable candidate member list. Our catalog enables the determination of structural and kinematic parameters of the cluster and other properties of its stellar population.

Methods. 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. The membership probabilities were determined using a probabilistic random forest algorithm and were further refined by requiring positions in the Hertzsprung-Russel diagram consistent with a young age. Based on a list of 1030 probable members, we estimate that the distance to Berkeley 59 is 1009 ± 12 pc. Masses, extinction, and ages were derived by fitting the spectral energy distributions to atmospheric and evolutionary models, and internal dynamics was analyzed using proper motions relative to the mean motion of the cluster.

Results. Berkeley 59 exhibits an asymmetric expansion pattern. The velocity increases outward, and the preferred motion is toward the north. The initial mass function between 0.4 and 7 M_⊙ follows a single power law (dN/dM ∝ M^−α) with a slope α = 2.3 ± 0.3, which is consistent with Salpeter’s slope and previous studies in the region. We estimated the median age of the region based on the Hertzsprung-Russel diagram as 2.9 Myr. The velocity dispersion of Berkeley 59 exceeds the virial velocity dispersion derived from its total mass (650 ± 30 M_⊙) and half-mass radius (1.71 ± 0.13 pc). The two-dimensional motions of a stellar group located ~1° north of Berkeley 59 provide further support for the previously proposed scenario of triggered star formation.