Detailed 3D structure of OrionA in dust with Gaia DR2
  The unprecedented astrometry from Gaia DR2 provides us with an opportunity to
study in detail molecular clouds in the solar neighbourhood. Extracting the
wealth of information in these data remains a challenge, however. We have
further improved our Gaussian Processes-based, three-dimensional dust mapping
technique to allow us to study molecular clouds in more detail. These
improvements include a significantly better scaling of the computational cost
with the number of stars, and taking into account distance uncertainties to
individual stars. Using Gaia DR2 astrometry together with 2MASS and WISE
photometry for 30 000 stars, we infer the distribution of dust out to 600 pc in
the direction of the Orion A molecular cloud. We identify a bubble-like
structure in front of Orion A, centred at a distance of about 350 pc from the
Sun. The main Orion A structure is visible at slightly larger distances, and we
clearly see a tail extending over 100 pc that is curved and slightly inclined
to the line-of-sight. The location of our foreground structure coincides with
5-10 Myr old stellar populations, suggesting a star formation episode that
predates that of the Orion Nebula Cluster itself. We identify also the main
structure of the Orion B molecular cloud, and in addition discover a background
component to this at a distance of about 460 pc from the Sun. Finally, we
associate our dust components at different distances with the plane-of-the-sky
magnetic field orientation as mapped by Planck. This provides valuable
information for modelling the magnetic field in 3D around star forming regions.
