Rotation and Lithium Confirmation of a 500 Parsec Halo for the Open
  Cluster NGC 2516
  Recent analyses of the Gaia data have identified 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 ($\approx$150
Myr), which has a classical tidal radius of 10 pc and an apparent halo of stars
spanning 500 pc ($20^\circ$ on-sky). 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 the same age as the cluster's 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. The lithium abundances of
stars in the halo are higher than in the field, and are correlated with the
stellar rotation rate and binarity fraction, as has been noted in other young
open clusters. Broadly speaking, this work supports a new paradigm wherein the
halos of open clusters are often more populous than their cores. We highlight
implications for spectroscopic survey targeting, open cluster dispersal, and
planet searches around young stars.
