Dynamical state of four open clusters

¹ School of Science, Hunan Institute of Technology, Hengyang 421002, China
² School of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, China
³ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
⁴ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 101408, China

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 23 April 2025
Accepted: 17 March 2026

Abstract

Context. Open clusters are vital laboratories for studying stellar dynamics and evolution, with mass segregation – the preferential concentration of massive stars toward cluster cores – serving as a key indicator of internal dynamical processes. This study investigates four open clusters (NGC 7243, NGC 2301, NGC 1528, and NGC 2281; ages 100–650 Myr) using deep multicolor photometry from the Beijing-Arizona-Taiwan-Connecticut (BATC) Sky Survey, combined with Gaia Data Release 3 (DR3), to explore their mass functions and dynamical states.

Aims. The work seeks to characterize mass segregation patterns across spatial scales to establish its origin (primordial vs. dynamical) and to provide the first conclusive evidence of mass segregation in NGC 2281. Additionally, we analyze how cluster relaxation timescales and evolutionary stages influence their dynamical architectures.

Methods. Fundamental parameters were derived via Bayesian methods. The tidal radii were determined from the intersection of the observed cumulative mass profiles and theoretical tidal mass curves, and mass functions were analyzed through power-law fits (Φ(m) ∝ m^α) performed separately for the inner (bound) and outer (tidal) regions.

Results. All clusters exhibit signs of mass segregation. However, only NGC 2281 shows a statistically significant steepening of the mass function slope in its outer tidal region, indicating significant evaporation of low-mass stars and providing the first clear evidence of strong, dynamically evolved mass segregation in this cluster. For NGC 7243, NGC 2301, and NGC 1528, the derived τ values (τ = Age/t_relax > 3) suggest advanced dynamical evolution, yet their outer regions contain too few stars to robustly confirm the expected steepening of the mass function.