3D shape of Orion A from Gaia DR2

TL;DR

This study reveals that Orion A is a large, cometary-shaped molecular cloud with a 3D extent of about 90 pc, significantly larger than its 2D projection suggests, and has a structure similar to Milky Way filaments.

Contribution

The paper provides the first 3D shape and orientation of Orion A using Gaia DR2 data, showing it is a giant, cometary-like cloud with a complex structure.

Findings

Orion A is a 90 pc long cometary-shaped cloud, not a 40 pc filament.

It has a dense, star-forming head and a lower density tail.

Its aspect ratio and structure resemble large Milky Way filaments.

Abstract

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.