Gaia DR2 unravels incompleteness of nearby cluster population: New open clusters in the direction of Perseus

TL;DR

Using Gaia DR2 data, the study systematically discovers 41 new open clusters near Perseus, challenging the notion that the nearby open cluster population is nearly complete within 1.8 kpc.

Contribution

The paper introduces a novel multi-step method combining spatial, proper motion, and membership analyses to identify new open clusters using Gaia DR2 data.

Findings

Discovered 41 new open clusters in the Perseus region.

Revealed that the nearby open cluster population is significantly incomplete.

Identified NGC 886 as a true cluster, not an asterism.

Abstract

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.