The extended halo of NGC 2682 (M 67) from Gaia DR2
  Context: NGC 2682 is a nearby open cluster, approximately 3.5 Gyr old.
Dynamically, most open clusters should dissolve on shorter timescales, of ~ 1
Gyr. Having survived until now, NGC 2682 was likely much more massive in the
past, and is bound to have an interesting dynamical history. Aims: We
investigate the spatial distribution of NGC 2682 stars to constrain its
dynamical evolution, especially focusing on the marginally bound stars in the
cluster outskirts. Methods: We use Gaia DR2 data to identify NGC 2682 members
up to a distance of ~150 pc (10 degrees). Two methods (Clusterix and UPMASK)
are applied to this end. We estimate distances to obtain three-dimensional
stellar positions using a Bayesian approach to parallax inversion, with an
appropriate prior for star clusters. We calculate the orbit of NGC 2682 using
the GRAVPOT16 software. Results: The cluster extends up to 200 arcmin (50 pc)
which implies that its size is at least twice as previously believed. This
exceeds the cluster Hill sphere based on the Galactic potential at the distance
of NGC 2682. Conclusions: The extra-tidal stars in NGC 2682 may originate from
external perturbations such as disk shocking or dynamical evaporation from
two-body relaxation. The former origin is plausible given the orbit of NGC
2682, which crossed the Galactic disk ~40 Myr ago.
