Discovery of Tidal Tails around the Old Open Cluster NGC 2506

Abstract

We report the discovery of tidal tails around the old open cluster NGC 2506 based on Gaia-DR2. A principal-component analysis and Gaussian mixture model are used to identify extra-tidal stars. We identify 2301 stars closely related to the cluster, 147 of which are likely extra-tidal stars. We find clear evidence for the existence of two well-defined tidal tails emerging from the cluster. The tidal tails extend out to projected distances of at least 54 pc, which is nearly three times as large as the tidal radius of the cluster. Furthermore, we find that the extra-tidal stars have a wide mass spectrum (∼0.6-1.4 M_⊙), which cannot be completely explained by mass segregation and the preferential evaporation of low-mass stars. An extra dynamical process such as disk-shocking may also play an important role in forming these extra-tidal stars. A mass-loss rate of ∼5.3 M_⊙ Myr^-1 is estimated for the cluster. This reveals that NGC 2506 is currently undergoing heavy mass-loss due to star escapes.