2025ApJ...981L..34D

A successful theory of star formation should predict the number of objects as a function of their mass produced through star-forming events. Previous studies in star-forming regions and the solar neighborhood have identified a mass function increasing from the hydrogen-burning limit down to about 10 M_J. Theory predicts a limit to the fragmentation process, providing a natural turnover in the mass function down to the opacity limit of turbulent fragmentation, thought to be near 1-10 M_J. Programs to date have not been sensitive enough to probe the hypothesized opacity limit of fragmentation. We present the first identification of a turnover in the initial mass function below 12 M_J within NGC 2024, a young star-forming region. With JWST/NIRCam deep exposures across 0.7-5 μm, we identified several free-floating objects down to roughly 3 M_J with sensitivity to 0.5 M_J. We present evidence for a double power-law model increasing from about 60 M_J to roughly 12 M_J, consistent with previous studies, followed by a decrease down to 0.5 M_J. Our results support the predictions of star and brown dwarf formation theory, identifying the theoretical turnover in the mass function and suggesting the fundamental limit of turbulent fragmentation to be near 3 M_J.