Gaia DR2 unravels incompleteness of nearby cluster population: new open clusters in the direction of Perseus^★

¹ Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
e-mail: tcantat@fqa.ub.edu
² CENTRA, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
³ Department of Computer Science, University of Freiburg, Georges-Koehler-Allee 052, 79110 Freiburg, Germany
⁴ Department of Physics, McWilliams Center for Cosmology, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, USA
⁵ Department of Physics & Astronomy, University of North Carolina at Chapel Hill, NC 27599-3255, USA
⁶ Department of Physics and Institute for Theoretical and Computational Physics, University of Crete, 71003 Heraklion, Greece
⁷ Foundation for Research and Technology – Hellas, IESL, Voutes, 7110 Heraklion, Greece
⁸ Center for Cosmology and Particle Physics, New York University, 726 Broadway, New York, NY 10004, USA
⁹ Department of Physics, New York University, 726 Broadway, New York, NY 10004, USA
¹⁰ Instituto Federal de Educação, Ciência e Tecnologia de Pernambuco, IFPE, 50670-430, Recife-PE, Brazil
¹¹ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹² Université Clermont Auvergne, CNRS/IN2P3, LPC, 63000 Clermont-Ferrand, France
¹³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹⁴ LAL, Univ. Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay, France
¹⁵ Paris-Saclay Center for Data Science, Université Paris-Saclay, Orsay, France
¹⁶ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal

Received: 18 October 2018
Accepted: 14 March 2019

Abstract

Context. Open clusters (OCs) are popular tracers of the structure and evolutionary history of the Galactic disc. 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.

Aims. We perform a systematic search for new OCs in the direction of Perseus using precise and accurate astrometry from Gaia DR2.

Methods. We implemented a coarse-to-fine search method. First, we exploited 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 employed a Gaussian mixture model in the proper motion space to tag fields quickly around OC candidates. Thirdly, we applied an unsupervised membership assignment method, UPMASK, to scrutinise the candidates. We visually inspected colour-magnitude diagrams to validate the detected objects. Finally, we performed a diagnostic to quantify the significance of each identified over-density in proper motion and in parallax space.

Results. 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 study challenges the previous claim of a near-complete sample of OCs up to 1.8 kpc. Our results reveal that this claim requires revision, and a complete census of nearby OCs is yet to be found.