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.
تحميل البحث