The Kinematics of the Scorpius-Centaurus OB Association from Gaia DR1
  We present a kinematic study of the Scorpius-Centaurus (Sco-Cen) OB
association (Sco OB2) using Gaia DR1 parallaxes and proper motions. Our goal is
to test the classical theory that OB associations are the expanded remnants of
dense and compact star clusters disrupted by processes such as residual gas
expulsion. Gaia astrometry is available for 258 out of 433 members of the
association, with revised Hipparcos astrometry used for the remainder. We use
this data to confirm that the three subgroups of Sco-Cen are gravitationally
unbound and have non-isotropic velocity dispersions, suggesting they have not
had time to dynamically relax. We also explore the internal kinematics of the
subgroups to search for evidence of expansion. We test Blaauw's classical
linear model of expansion, search for velocity trends along the Galactic axes,
compare the expanding and non-expanding convergence points, perform traceback
analysis assuming both linear trajectories and using an epicycle approximation,
and assess the evidence for expansion in proper motions corrected for virtual
expansion / contraction. None of these methods provide coherent evidence for
expansion of the subgroups, with no evidence to suggest that the subgroups had
a more compact configuration in the past. We find evidence for kinematic
substructure within the subgroups that supports the view that they were not
formed by the disruption of individual star clusters. We conclude that Sco-Cen
was likely born highly substructured, with multiple small-scale star formation
events contributing to the overall OB association, and not as single,
monolithic bursts of clustered star formation.
