Recovering extra-tidal open cluster members via multi-elemental chemical tagging
The identification of open cluster (OC) members has been revolutionized by high-precision Gaia astrometry, yet traditional kinematic membership selections remain inherently conservative, often overlooking stars in tidal tails or those with perturbed velocities. This study investigates the reliability of these kinematic probabilities by searching for leaky cluster members -- stars that fail standard kinematic membership criteria ($P < 0.7$) but possess chemical signatures identical to their host clusters. Using high-resolution spectroscopic data from the Gaia-ESO Survey, we established a seven-element chemical fingerprint ([Fe/H], Li, Si, Ca, Ti, Co, and Ni) for 34 OCs. We identified a sample of 63 stars across 22 clusters that are chemically indistinguishable from their host populations despite being kinematically rejected by standard algorithms. By cross-referencing these targets with Jacobi radii (rJ) derived from modern Milky Way potential models, we find that 35% are located in extra-tidal regions (d > rJ), providing direct evidence of active cluster dissolution and tidal debris. The remaining 65% are located within the Jacobi radius, suggesting that their kinematic rejection is likely due to orbital motion in unresolved binary systems. These results demonstrate that chemical tagging is a critical tool for overcoming the spatial and kinematic biases of astrometric catalogues. By recovering these lost members, we provide a more complete census of cluster mass loss and underscore the necessity of a hybrid chemical-kinematic approach to map the transition of stars from bound systems to the Galactic field.