3D mapping of young stars in the solar neighbourhood with Gaia DR2
  We study the three dimensional arrangement of young stars in the solar
neighbourhood using the second release of the Gaia mission (Gaia DR2) and we
provide a new, original view of the spatial configuration of the star forming
regions within 500 pc from the Sun. By smoothing the star distribution through
a gaussian filter, we construct three dimensional density maps for early-type
stars (upper-main sequence, UMS) and pre-main sequence (PMS) sources. The PMS
and the UMS samples are selected through a combination of photometric and
astrometric criteria. A side product of the analysis is a three dimensional,
G-band extinction map, which we use to correct our colour-magnitude diagram for
extinction and reddening. Both density maps show three prominent structures,
Scorpius-Centaurus, Orion, and Vela. The PMS map shows a plethora of lower mass
star forming regions, such as Taurus, Perseus, Cepheus, Cassiopeia, and
Lacerta, which are less visible in the UMS map, due to the lack of large
numbers of bright, early-type stars. We report the finding of a candidate new
open cluster towards $l, b \sim 218.5^{\circ}, -2^{\circ}$, which could be
related to the Orion star forming complex. We estimate ages for the PMS sample
and we study the distribution of PMS stars as a function of their age. We find
that younger stars cluster in dense, compact clumps, and are surrounded by
older sources, whose distribution is instead more diffuse. The youngest groups
that we find are mainly located in Scorpius-Centaurus, Orion, Vela, and Taurus.
Cepheus, Cassiopeia, and Lacerta are instead more evolved and less numerous.
Finally, we find that the three dimensional density maps show no evidence for
the existence of the ring-like structure which is usually referred to as the
Gould Belt.
