The substellar population in Corona Australis

¹ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisbon, Portugal
² Departamento de Física, Faculdade de Ciências, Universidade de Lisboa, Edifício C8, Campo Grande, 1749-016 Lisbon, Portugal
³ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁴ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
⁵ European Space Research and Technology Centre (ESTEC), European Space Agency, Postbus 299, 2200AG Noordwijk, The Netherlands
⁶ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁷ University of Split, Faculty of Science, Rud¯era Boškovic´a 33, 21000 Split, Croatia
⁸ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁹ Department of Physics and Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA

^★ Corresponding author: kmuzic@fc.ul.pt

Received: 11 June 2025
Accepted: 28 July 2025

Abstract

Context. The substellar initial mass function (IMF) and the formation mechanisms of brown dwarfs (BDs) remain key open questions in star formation theory. A detailed census and characterization of the IMF in a large number of star-forming regions are essential for constraining these processes.

Aims. We identify and spectroscopically confirm very low-mass members of the Corona Australis (CrA) star-forming region to refine its substellar census, determine its low-mass IMF, and compare it to other clusters.

Methods. Using deep I-band photometry from Suprime-Cam/Subaru and data from the VISTA Hemisphere Survey (VHS), we identified low-mass BD candidates in CrA. We subsequently conducted near-infrared spectroscopic follow-up of 173 of these candidates with KMOS/VLT, and we also obtained optical spectra for eight kinematic candidate members identified via Gaia data using FLOYDS/LCO.

Results. The kinematic candidates observed with optical spectroscopy are confirmed as low-mass stellar members with spectral types M1 to M5. In contrast, all 173 BD candidates observed with KMOS are identified as contaminants. Although the follow-up yielded no new substellar members, it places strong constraints on the number of undetected substellar objects in the region. Combined with literature data, this enables us to derive the substellar IMF, which is consistent with a single power-law slope of α = 0.95 ± 0.06 in the range 0.01–1 M_⊙ or α = 0.33 ± 0.19 in the range 0.01–0.1 M_⊙. The star-to-BD ratio in CrA is ∼2. We also provide updated IMFs and star-to-BD ratios for Lupus 3 and Cha I from the SONYC survey, reflecting revised distances from Gaia. Finally, we estimate surface densities and median far-ultraviolet fluxes for six star-forming regions and clusters to characterize their environments and compare their substellar populations as a function of environmental properties.

Conclusions. The IMF and star-to-BD ratio show no clear dependence on stellar density or ionizing flux from the massive stars. A combined effect in which one factor enhances and the other suppresses BD formation also appears unlikely.