Extended stellar systems in the solar neighborhood -- V. Discovery of
  coronae of nearby star clusters
  In this paper, we present a novel view on the morphology and the dynamical
state of 10 prominent, nearby ($\leq$ 500 pc), and young ($\sim$30-300 Myr)
open star clusters with Gaia DR2: $\alpha\,$Per, Blanco 1, IC 2602, IC 2391,
Messier 39, NGC 2451A, NGC 2516, NGC 2547, Platais 9, and the Pleiades. We
introduce a pioneering member identification method that is informed by cluster
bulk velocities and deconvolves the spatial distribution with a mixture of
Gaussians. Our approach enables inferring the clusters' true spatial
distribution by effectively filtering field star contaminants while at the same
time mitigating the impact of positional errors along the line of sight. This
first application of the method reveals the existence of vast stellar coronae,
extending for $\gtrsim\,$100 pc and surrounding the, by comparison tiny and
compact, cluster cores. The coronae and cores form intertwined, co-eval, and
co-moving extended cluster populations, each encompassing tens of thousands of
cubic parsec and stretching across tens of degrees on the sky. Our analysis
shows that the coronae are gravitationally unbound but largely comprise the
bulk of the populations' stellar mass. Most systems are in a highly dynamic
state, showing evidence of expansion and sometimes simultaneous contraction
along different spatial axes. The velocity field of the extended populations
for the cluster cores appears asymmetric but is aligned along a spatial axis
unique to each cluster. The overall spatial distribution and the kinematic
signature of the populations are largely consistent with the differential
rotation pattern of the Milky Way. This finding underlines the important role
of global Galactic dynamics to the fate of stellar systems. Our results
highlight the complexity of the Milky Way's open cluster population and call
for a new perspective on the characterization and dynamical state of open
clusters.
