Gaia DR2 unravels incompleteness of nearby cluster population: New open
  clusters in the direction of Perseus
  Open clusters (OCs) are popular tracers of the structure and evolutionary
history of the Galactic disk. The OC population is often considered to be
complete within 1.8 kpc of the Sun. The recent Gaia Data Release 2 (DR2) allows
the latter claim to be challenged. We perform a systematic search for new OCs
in the direction of Perseus using precise and accurate astrometry from Gaia
DR2. We implement a coarse-to-fine search method. First, we exploit spatial
proximity using a fast density-aware partitioning of the sky via a k-d tree in
the spatial domain of Galactic coordinates, (l, b). Secondly, we employ a
Gaussian mixture model in the proper motion space to quickly tag fields around
OC candidates. Thirdly, we apply an unsupervised membership assignment method,
UPMASK, to scrutinise the candidates. We visually inspect colour-magnitude
diagrams to validate the detected objects. Finally, we perform a diagnostic to
quantify the significance of each identified overdensity in proper motion and
in parallax space We report the discovery of 41 new stellar clusters. This
represents an increment of at least 20% of the previously known OC population
in this volume of the Milky Way. We also report on the clear identification of
NGC 886, an object previously considered an asterism. This letter challenges
the previous claim of a near-complete sample of open clusters up to 1.8 kpc.
Our results reveal that this claim requires revision, and a complete census of
nearby open clusters is yet to be found.
