The Formation of a Stellar Association in the NGC 7000/IC 5070 Complex:
  Results from Kinematic Analysis of Stars and Gas
  We examine the clustering and kinematics of young stellar objects (YSOs) in
the North America/Pelican Nebulae, as revealed by Gaia astrometry, in relation
to the structure and motions of the molecular gas, as indicated in molecular
line maps. The Gaia parallaxes and proper motions allow us to significantly
refine previously published lists of YSOs, demonstrating that many of the
objects previously thought to form a distributed population turn out to be
non-members. The members are subdivided into at least 6 spatio-kinematic
groups, each of which is associated with its own molecular cloud component or
components. Three of the groups are expanding, with velocity gradients of
0.3-0.5 km/s/pc, up to maximum velocities of ~8 km/s away from the groups'
centers. The two known O-type stars associated with the region, 2MASS
J20555125+4352246 and HD 199579, are rapidly escaping one of these groups,
following the same position-velocity relation as the low-mass stars. We
calculate that a combination of gas expulsion and tidal forces from the clumpy
distribution of molecular gas could impart the observed velocity gradients
within the groups. However, on a global scale, the relative motions of the
groups do not appear either divergent or convergent. The velocity dispersion of
the whole system is consistent with the kinetic energy gained due to
gravitational collapse of the complex. Most of the stellar population has ages
similar to the free-fall timescales for the natal clouds. Thus, we suggest the
nearly free-fall collapse of a turbulent molecular cloud as the most likely
scenario for star formation in this complex.
