Gaia: Orion's Integral Shaped Filament is a Standing Wave
  The Integral Shaped Filament (ISF) is the nearest molecular cloud with rapid
star formation, including massive stars, and it is therefore a star-formation
laboratory. We use Gaia parallaxes, to show that the distances to young Class
II stars ('disks') projected along the spine of this filament are related to
the gas radial velocity by $$ v = -{D\over\tau} + K;\qquad \tau = 4\,{\rm Myr},
$$ where $K$ is a constant. This implies that the ISF is a standing wave, which
is consistent with the Stutz & Gould (2016) 'Slingshot' prediction. The
$\tau=4\,{\rm Myr}$ timescale is consistent with the 'Slingshot' picture that
the Orion Nebula Cluster (ONC) is the third cluster to be violently split off
from the Orion A cloud (following NGC 1981 and NGC 1987) at few-Myr intervals
due to gravito-magnetic oscillations. We also present preliminary evidence that
the truncation of the ISF is now taking place $16^\prime$ south of the ONC and
is mediated by a torsional wave that is propagating south with a characteristic
timescale $\tau_{\rm torsion} = 0.5\,{\rm
  Myr}$, i.e. eight times shorter. The relation between these two wave
phenomena is not presently understood.
