Substellar population of the young massive cluster RCW 36 in Vela^★

¹ Instituto de Astrofĩisica e Ciências do Espaço (IA), Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
² Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8, Campo Grande, 1749-016 Lisbon, Portugal
³ Laboratoire d’Astrophysique de Bordeaux (LAB), Université de Bordeaux, Bât., B18N, Allée Geoffroy Saint-Hilaire CS 50023, 33615 PESSAC CEDEX, France
⁴ Institut universitaire de France (IUF), 1 rue Descartes, 75231 Paris CEDEX 05, France
⁵ NSF NOIRLab/Vera C. Rubin Observatory, Casilla 603, La Serena, Chile
⁶ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁸ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
⁹ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
¹⁰ Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, NY 10024, USA
¹¹ Instituto de Estudios Astrofisicos, Facultad de Ingenieriıa y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹² Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
¹³ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
¹⁴ Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, HI96743, USA
¹⁵ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 September 2025
Accepted: 8 December 2025

Abstract

Context. The shape of the initial mass function (IMF) remains a fundamental yet contentious topic in the study of stellar formation and evolution. It is imperative to understand the potential variability of the IMF across different young regions. This study examines the IMF within the young massive cluster RCW 36 situated in the Vela Molecular Ridge, comparable with the Orion Nebula Cluster in terms of stellar surface density.

Aims. The primary objective of this research is to construct the most comprehensive census of the stellar population in RCW 36 to date and determine the first ever IMF and star to brown dwarf (BD) ratio for the cluster.

Methods. We used state-of-the art observational techniques, drawing on new GLAO observations conducted with HAWK-I/VLT in addition to archival data from 2MASS, SOFI/NTT, and new kinematics from Gaia DR3. To enhance photometric accuracy and source extraction, we employed DENEB, an advanced deep learning algorithm capable of removing the complex filamentary nebula in our images. Statistical comparisons of color-magnitude diagrams were performed between RCW 36 and a control field, also obtained using HAWK-I under the same mode, to assign membership weights for the sources in our field. Mass estimates to individual sources were also derived through comparison with model isochrones in order to determine the IMF using the membership weights.

Results. We found a new distance of 954 ± 40 pc. We determined the IMF for RCW 36 down to ~0.03 M_⊙, characterized by a broken power law (dN/d M ∝ M^−α) with α = 1.62 ± 0.03 (0.20 M_⊙ −20 M_⊙) and α = 0.46 ± 0.14 (0.03 M_⊙–0.20 M_⊙). We also determined the star-BD ratio to be 2–5, in agreement with other Galactic clusters. Lastly, through a study of the differences in the IMF within and outside 0.2 pc and the cumulative mass distributions for low-mass and intermediate to high-mass sources, we also detected signs of possible mass segregation within RCW 36, which should be primordial.

Conclusions. RCW 36 shares many characteristics with other young massive clusters, such as a shallower than Salpeter high-mass slope and the possibility of mass segregation. The flatter lower-mass regime of the IMF is similar to most Galactic clusters. The star-BD ratio is also in line with the observed values in other clusters, independent of their inherent properties.