An Open Cluster Spread Across 500 Parsecs

Recent analyses of the Gaia data have reported the existence of diffuse stellar populations surrounding nearby open clusters. It is important to verify that these “halos”, “tails”, and “strings” are of similar ages and compositions as stars in the denser part of the cluster. We present an analysis of NGC 2516 (150 Myr), which has a classical tidal radius of 10 pc and an apparent halo of stars spanning 500 pc. Combining photometry from Gaia, rotation periods from TESS, and lithium measurements from Gaia-ESO and GALAH, we find that the halo of NGC 2516 is real and coeval with the cluster core. Two thirds of kinematically selected halo members out to 250 pc from the cluster center have rotation periods consistent with a gyrochronological age of 150 Myr. A comparison sample of field stars shows no such trend. In addition, the lithium abundances of stars in the halo are elevated relative to the field, and are correlated with the stellar rotation rate in a manner consistent with observations of the Pleiades and Psc Eri. This work expands the set of confirmed NGC 2516 members by a factor of two, and suggests that the halos of many open clusters are more populous than their cores. The implications for spectroscopic survey targeting, open cluster dispersal, and planet searches around young stars will be highlighted.