Untangling the Galaxy. II. Structure within 3 kpc
  We present the results of the hierarchical clustering analysis of the Gaia
DR2 data to search for clusters, co-moving groups, and other stellar
structures. The current paper builds on the sample from the previous work,
extending it in distance from 1 kpc to 3 kpc, increasing the number of
identified structures up to 8292. To aid in the analysis of the population
properties, we developed a neural network called Auriga to robustly estimate
the age, extinction, and distance of a stellar group based on the input
photometry and parallaxes of the individual members. We apply Auriga to derive
the properties of not only the structures found in this paper, but also
previously identified open clusters. Through this work, we examine the temporal
structure of the spiral arms. Specifically, we find that the Sagittarius arm
has moved by >500 pc in the last 100 Myr, and the Perseus arm has been
experiencing a relative lull in star formation activity over the last 25 Myr.
We confirm the findings from the previous paper on the transient nature of the
spiral arms, with the timescale of transition of a few 100 Myr. Finally, we
find a peculiar ~1 Gyr old stream of stars that appears to be heliocentric. It
is unclear what is the origin of it.
