An initial overview of the extent and structure of recent star formation
  within the Serpens Molecular Cloud using Gaia Data Release 2
  The dense clusters within the Serpens Molecular Cloud are among the most
active regions of nearby star formation. In this paper, we use Gaia DR2
parallaxes and proper motions to statistically measure 1167 kinematic members
of Serpens, few of which were previously identified, to evaluate the star
formation history of the complex. The optical members of Serpens are
concentrated in three distinct groups located at 380 to 480 pc; the densest
clusters are still highly obscured by optically-thick dust and have few optical
members. The total population of young stars and protostars in Serpens is at
least 2000 stars, including past surveys that were most sensitive to protostars
and disks, and may be far higher. Distances to dark clouds measured from
deficits in star counts are consistent with the distances to the optical star
clusters. The Serpens Molecular Cloud is seen in the foreground of the Aquila
Rift, dark clouds located at 600 to 700 pc, and behind patchy extinction, here
called the Serpens Cirrus, located at ~250 pc. Based on the lack of a
distributed population of older stars, the star formation rate throughout the
Serpens Molecular Cloud increased by at least a factor of 20 within the past ~5
Myr. The optically bright stars in Serpens Northeast are visible because their
natal molecular cloud has been eroded and not because they were flung outwards
from a central factory of star formation. The separation between subclusters of
20 to 100 pc and the absence of an older population leads to speculation that
an external forcing was needed to trigger the active star formation.
