Unveiling the binary nature of NGC 2323

¹ Astrophysics Division, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
² School of Astronomy and Space Science, University of Chinese Academy of Sciences, No. 19A, Yuquan Road, Beijing 100049, PR China
³ Institut de Ciències del Cosmos, Universitat de Barcelona (ICCUB), Martí i Franquès 1, 08028 Barcelona, Spain
⁴ School of Physics and Astronomy, Sun Yat-sen University, Daxue Road, Zhuhai 519082, China
⁵ CSST Science Center for the Guangdong-Hong Kong-Macau Greater Bay Area, Zhuhai 519082, China
⁶ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁷ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), C Martí i Franquès 1, 08028 Barcelona, Spain
⁸ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
⁹ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, China
¹⁰ School of Physics and Astronomy, China West Normal University, No. 1 Shida Road, Nanchong 637002, China

^★ Corresponding authors; qinsongmei@shao.ac.cn; jzhong@shao.ac.cn

Received: 11 November 2025
Accepted: 17 December 2024

Abstract

Context. As a well-known open cluster, NGC 2323 (also called M50) has been widely investigated for over a hundred years and has always been considered a classical single cluster.

Aims. We studied the binary structure nature of NGC 2323 with Gaia Data Release 3 (DR3).

Methods. We discovered the binary structure of NGC 2323 by the HDBSCAN algorithm based on the astrometric and photometric data from Gaia DR3. Then we employed the PETAR N-body code to investigate its binary structure origin.

Results. We found that NGC 2323 consists of two distinct subclusters (NGC 2323-a and NGC 2323-b) that are still gravitationally bound. They have very close positions (three-dimensional Δpos = 12.3 pc, σ_Δpos = 3.4 pc) and similar tangential velocities (two-dimensional ΔV = 2.2 km s⁻¹, σ_ΔV = 0.02 km s⁻¹). The best isochrone fitting ages of the two clusters are the same (158 Myr), further proving their possibly common origin. The numerical N-body simulation suggests that the less massive cluster is unlikely to be the cluster tidal tails created by the differential rotation of the Milky Way.