C. Ordenes-Huanca, A. Bayo, M. G. Guarcello, M. Zoccali, Á. Rojas-Arriagada, V. Almendros-Abad, M. Andersen, R. Bonito, V. Guzmán, E. Moraux, K. Muzic, L. Prisinzano, S. Sciortino, N. Wright
Abstract
Context. Westerlund 1 (Wd1), the most massive young star cluster in the Milky Way, is an excellent laboratory for studying star formation and early stellar evolution in a starburst-like environment. However, high extinction restricts studies of its stellar content, and a focus on high-mass stars limits our knowledge of the full spatial extent of the cluster. Aims. We characterize the near-infrared (NIR) variability of the stellar population of Wd1, filling the mass gap between massive stars traced by Gaia and very low-mass stars from previous Extended Westerlund 1 and 2 Open Clusters Survey (EWOCS) studies, to provide a more complete view of cluster membership across solar and super-solar masses. Methods. We exploited data from the VISTA Variables of the Vía Láctea survey and its extension (VVVX), using NIR point-spread-function photometry and astrometric solutions from its latest data release, namely the VIRAC2 catalogs, mainly in the Ks band. Their large spatial coverage enables the study of both the central regions and outskirts of the cluster. We applied the HDBSCAN clustering algorithm in a 6D parameter space to differentiate cluster members from field contaminants, assessing robustness through Monte Carlo simulations. Variable sources along the line of sight were also identified and characterized. Results. We identify 1286 high-probability candidate members (12 <= J <= 18 mag) spanning ~1.5--20 M⊙, adopting both PARSEC 5 and 6 Myr isochrones (AKs = 0.6, d = 4.23 kpc). A considerable fraction (~34%) show statistically significant flux variations. We present, for the first time, a parametric analysis of variability modes of Wd1 candidate members in the Ks band, providing a membership catalog suitable for future kinematic studies.
Keywords
stars: kinematics and dynamics / stars: variables: general / open clusters and associations: individual: Westerlund 1
Astronomy & Astrophysics
Volume 712, Article Number A193, Number of pages 17
2026 August
