Gaia: Orion's Integral Shaped Filament is a Standing Wave

Abstract

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.