3D shape of Orion A from Gaia DR2

Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.We use the 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 ~75 pc long Tail. The true extent of Orion A is not the projected ~40 pc but ~90 pc, making it by far the largest molecular cloud in the local neighborhood. Its aspect ratio (~30:1) and high column-density fraction (~45%) 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.Peer reviewedFinal Accepted Versio

3D dynamics of the Orion cloud complex

We present the first study of the three-dimensional (3D) dynamics of the gas in the entire southern Orion cloud complex. We used the parallaxes and proper motions of young stellar objects (YSOs) from Gaia DR2 as a proxy for gas distance and proper motion, and the gas radial velocities from archival CO data, to compute the space motions of the different star-forming clouds in the complex, including subregions in Orion A, Orion B, and two outlying cometary clouds. From the analysis of the clouds’ orbits in space and time, we find that they were closest about 6 Myr ago and are moving radially away from roughly the same region in space. This coherent 100-pc scale radial motion supports a scenario where the entire complex is reacting to a major feedback event, which we name the Orion-BB (big blast) event. This event, which we tentatively associate with the recently discovered Orion X stellar population, shaped the distribution and kinematics of the gas we observe today, although it is unlikely to have been the sole major feedback event in the region. We argue that the dynamics of most of the YSOs carry the memory of the feedback-driven star formation history in Orion and that the majority of the young stars in this complex are a product of large-scale triggering, which can raise the star formation rate by at least an order of magnitude, as for the head of Orion A (the Integral Shape Filament). Our results imply that a feedback, compression, and triggering process lies at the genesis of the Orion Nebula Cluster and NGC 2023/2024 in Orion B, thus confirming broadly the classical feedback-driven scenario proposed in Elmegreen & Lada (1977, ApJ, 214, 725). The space motions of the well-known young compact clusters, σ Orionis and NGC 1977, are consistent with this scenario. A momentum estimate suggests that the energy of a few to several supernovae is needed to power the coherent 3D gas motion we measure in this paper