Kinematic fingerprints of dynamical ejection: High-velocity blue straggler stars in the halo of NGC 188

¹ School of Artificial Intelligence, Yunnan Open University, Kunming 650599, China
² Center for Astrophysics, Guangzhou University, Guangzhou 510006, China

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

Received: 4 March 2026
Accepted: 23 April 2026

Abstract

Context. Open clusters are fundamental laboratories for studying stellar dynamics. In dynamically mature systems such as NGC 188 (~7 Gyr), mass segregation is expected to concentrate massive objects in the core, making the presence of high-mass stars in the halo a dynamical paradox.

Aims. We aim to characterize the blue straggler star (BSS) population in the extended halo of NGC 188 and investigate the potential dynamical mechanisms responsible for their presence in low-density regions (in the context of open clusters).

Methods. Utilizing Gaia DR3 astrometry and the UPMASK algorithm, we reconstructed a high-precision membership catalog. A robust differential reddening correction was applied to refine the color-magnitude diagram (CMD). We performed orbit back-integration and kinematic analysis to trace the trajectories of halo BSS candidates.

Results. We identified 1652 cluster members, including 45 BSSs cleanly separated into blue and red sequences. Most notably, we discovered two runaway BSSs in the distant halo (r > 2R_h) with relative velocities of 5.9–8.9 km s⁻¹, significantly exceeding the local escape velocity. Their velocity vectors show extreme radial alignment (cos θ ≥ 0.98) pointing back to the core. Orbit back-integration yields a recent ejection age of ~1.5 Myr.

Conclusions. These findings provide direct empirical evidence that the core of NGC 188 remains a “dynamical furnace” capable of ejecting BSSs via few-body scattering even at an age of 7 Gyr. Based on the observed flux, we estimate that such clusters act as a persistent “rejuvenation engine”, contributing a non-negligible population of young-looking stars to the Galactic field.