2025ApJ...991L..32A

The formation mechanism of brown dwarfs (BDs), whether akin to stars or ejected planetary-mass objects, remains debated. We present the first 3D radiation-MHD simulations of magnetized, turbulent, gravitationally unstable low-mass cores (0.05-0.1M_⊙) collapsing into proto-BDs. Using the RAMSES code with adaptive mesh refinement, we model the full dynamical range (10⁵-10²² cm^–3), including radiative transfer (flux limited diffusion) and nonideal MHD (ambipolar diffusion). Our simulations self-consistently follow the isothermal collapse, first hydrostatic core formation, H₂ dissociation, and BD birth. The resulting BDs have initial radii ≈0.75 R_⊙ and masses ≈0.8 M_Jup, growing via accretion as we follow the early evolution of the object. Crucially, we find that BDs may form similarly to low-mass stars but with a prolonged first-core phase, supporting a starlike formation scenario.