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arXiv · 2606.27623

EWOCS-VI: Probing the hidden intermediate-mass population of Westerlund 1

C. Ordenes-Huanca·A. Bayo·M. Guarcello·M. Zoccali·A. Rojas-Arriagada·V. Almendros-Abad·M. Andersen·R. Bonito·V. Guzmán·E. Moraux·K. Muzic·L. Prisinzano·S. Sciortino·N. J. Wright
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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 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 V\'ia L\'actea survey and its extension (VVVX), using NIR point spread function (PSF) photometry and astrometric solutions from its latest data release, namely the VIRAC2 catalogs, mainly in the Ks band. Their large spatial coverage enables study of both the central regions and outskirts of the cluster. We applied 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 $\sim 1.5$--$20\,M_{\odot}$, adopting both PARSEC 5 and 6 Myr isochrones ($A_{K_{\rm{s}}}=0.6\,\rm{mag}$, $d=4.23\,\rm{kpc}$). A considerable fraction (34%) shows 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.

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C. Ordenes-Huanca, A. Bayo, M. Guarcello, M. Zoccali, A. Rojas-Arriagada, V. Almendros-Abad, M. Andersen, R. Bonito, V. Guzmán, E. Moraux, K. Muzic, L. Prisinzano, S. Sciortino, N. J. Wright. 2026-06-26. EWOCS-VI: Probing the hidden intermediate-mass population of Westerlund 1. https://arxiv.org/abs/2606.27623

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