Rotation and Lithium Confirmation of a 500 Parsec Halo for the Open Cluster NGC 2516

TL;DR

This study confirms that the extended stellar halos around the open cluster NGC 2516 are of the same age and composition as the core, indicating they are part of the cluster and providing insights into cluster dispersal.

Contribution

It demonstrates that the halos of open clusters like NGC 2516 contain stars of the same age and composition as the core, supporting a new understanding of cluster structure.

Findings

Halo stars share the same age as the cluster core.

Halo stars exhibit higher lithium abundances than field stars.

Rotation periods in the halo are consistent with a 150 Myr age.

Abstract

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.