3D shape of Orion A from Gaia DR2
  We use the $\mathit{Gaia}$ DR2 distances of about 700 mid-infrared selected
young stellar objects in the benchmark giant molecular cloud Orion A to infer
its 3D shape and orientation. We find that Orion A is not the fairly straight
filamentary cloud that we see in (2D) projection, but instead a cometary-like
cloud oriented toward the Galactic plane, with two distinct components: a
denser and enhanced star-forming (bent) Head, and a lower density and
star-formation quieter $\sim$75 pc long Tail. The true extent of Orion A is not
the projected $\sim$40 pc but $\sim$90 pc, making it by far the largest
molecular cloud in the local neighborhood. Its aspect ratio ($\sim$30:1) and
high column-density fraction ($\sim45\%$) make it similar to large-scale Milky
Way filaments ("bones"), despite its distance to the galactic mid-plane being
an order of magnitude larger than typically found for these structures.
