2025A&A...701A.147M

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.