Disentangling the spatial substructure of Cygnus OB2 from Gaia DR2
  For the first time, we have explored the spatial substructure of the Cygnus
OB2 association using parallaxes from the recent second Gaia data release. We
find significant line-of-sight substructure within the association, which we
quantify using a parameterised model that reproduces the observed parallax
distribution. This inference approach is necessary due to the non-linearity of
the parallax-distance transformation and the asymmetry of the resulting
probability distribution. By using a Markov Chain Monte Carlo ensemble sampler
and an unbinned maximum likelihood test we identify two different stellar
groups superposed on the association. We find the main Cygnus OB2 group at
~1760 pc, further away than recent estimates have envisaged, and a foreground
group at ~1350 pc. We also calculate individual membership probabilities and
identify outliers as possible non-members of the association.
