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

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. 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). Utilizing 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. Gaia We identified 1,652 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 (̧os þeta 0.98) pointing back to the core. Orbit back-integration yields a recent ejection age of $∼1.5 Myr. 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.