Revisiting the Cygnus OB associations
  OB associations play an important role in Galactic evolution, though their
origins and dynamics remain poorly studied, with only a small number of systems
analysed in detail. In this paper we revisit the existence and membership of
the Cygnus OB associations. We find that of the historical OB associations only
Cyg OB2 and OB3 stand out as real groups. We search for new OB stars using a
combination of photometry, astrometry, evolutionary models and an SED fitting
process, identifying 4680 probable OB stars with a reliability of $>$90\%. From
this sample we search for OB associations using a new and flexible clustering
technique, identifying 6 new OB associations. Two of these are similar to the
associations Cyg OB2 and OB3, though the others bear no relationship to any
existing systems. We characterize the properties of the new associations,
including their velocity dispersions and total stellar masses, all of which are
consistent with typical values for OB associations. We search for evidence of
expansion and find that all are expanding, albeit anistropically, with stronger
and more significant expansion in the direction of Galactic longitude. We also
identify two large-scale (160 pc and 25 km s$^{-1}$) kinematic expansion
patterns across the Cygnus region, each including three of our new
associations, and attribute this to the effects of feedback from a previous
generation of stars. This work highlights the need to revisit the existence and
membership of the historical OB associations, if they are to be used to study
their properties and dynamics.
