Orion A with Gaia DR2: How 3D changes everything
Creators: name: Großschedl, Josefa
name: Alves, João
name: Meingast, Stefan
Publisher: name: Zenodo
Year: 2019
Subjects: star formation
astrometry
parallaxes
cloud topology
cloud motion
young stellar objects
Gaia
molecular clouds

description: The giant molecular cloud Orion A has been extensively studied in the past being the closest massive star-forming region to Earth (∼400 pc). It contains the famous Orion Nebula Cluster (ONC) toward the North, and the low-mass star-forming regions L1641 and L1647 toward the South, and it is the cluster and massive star formation benchmark, always assumed to be the straight 40 pc filament one sees from radio and sub-millimeter surveys. While performing an analysis of the gas mass distribution, and star formation rate across the cloud, we found that the gas is roughly distributed uniformly across the cloud (line-mass ∼ constant), while, oddly, the ONC region produced about a factor of ten more stars compared to the rest of the cloud. To try to understand this counter-intuitive relation between dense gas and star formation rate (SFR), we used Gaia DR2 data to reconstruct the 3D shape of the cloud, using the distances of young stellar objects as a proxy for cloud distances. We find that the ONC region indeed lies at about 400 pc, as expected, while the low-mass star-forming parts are, surprisingly, inclined about 70 deg from the plane of the sky, reaching about 500 pc towards the Southern end of the cloud. Orion A is then an approximately 90 pc long filament (twice as long as previously assumed), comet-shaped, with its "Head" (the ONC region) being “bent”. We propose that the peculiar shape might be explained by external feedback, also responsible for the observed increased SFR. Finally, by using Gaia proper motions and gas radial velocities (from CO data) we find that the cloud is currently located at a Z-minimum below the Galactic mid-plane. This new view of Orion A is an example of how Gaia is opening a new “3D window” on the topology of the dense star-forming ISM, a critical missing ingredient in our understanding of star formation.
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