Brown dwarfs have fascinated astronomers for decades: too massive to be planets but not massive enough to shine like stars, their origins remain debated.
Do they form like stars through gravitational collapse of gas clouds, or do they more closely resemble giant planets that have acquired extraordinary amounts of matter? A research team from CNRS Earth & Universe (see box) has just simulated for the first time the complete birth of a brown dwarf through gravitational collapse, revealing the physical properties of these mysterious objects in their early stages.
3D rendering showing the surface of the brown dwarf from simulation R1 at two weeks of age. The color scale indicates the intensity of the radial magnetic field at the surface. The green flux lines represent closed magnetic loops, i.e., field lines that loop from the north magnetic pole to the south magnetic pole at the surface. The red arrow indicates the axis of the brown dwarf's angular momentum. The left and bottom image panels are cross-sections of the brown dwarf's interior, showing the amplitude of the current density (left) and the specific entropy of the gas (bottom), respectively. The right image panel displays the maximum intensity of the magnetic field along the line of sight (y-axis). The mass and radius of the brown dwarf are displayed in the upper left corner. To achieve this result, the researchers used the RAMSES calculation code on French supercomputers at TGCC and CBPsmn. These cutting-edge 3D simulations integrate all key physical elements: gravity, radiation, and magnetic fields, interacting at different spatio-temporal scales.
The team modeled the collapse of low-mass dense cores (0.05 to 0.1 solar masses) over a considerable dynamic range, with 8 orders of magnitude in spatial extent (1000 astronomical units to a few thousand kilometers) and 17 orders of magnitude in densities (10
5 to 10
22 particles per centimeter
3). This self-consistent approach makes it possible to track the entire sequence: isothermal collapse, formation of the first hydrostatic core, dissociation of molecular hydrogen H₂, and finally the birth of the brown dwarf.
The results provide the most detailed picture to date of this formation. The resulting objects have initial radii of about 0.75 solar radii and masses of about 0.8 Jupiter masses, subsequently growing through accretion.
Crucially, the study reveals that brown dwarfs can form similarly to low-mass stars, but with a prolonged first core phase, thereby strengthening the stellar formation scenario. The simulations also demonstrate that the magnetic field embedded in the nascent object reaches about 1 kilogauss at the surface, with a mainly dipolar structure.
This work provides a theoretical framework for models aiming to describe the evolution of these very low-mass objects. The researchers now plan to analyze the birth of circumstellar disks around these nascent objects and study the evolution of their magnetic fields.
Visualization of magnetic fields inside a brown dwarf.
3D simulation of the birth of a brown dwarf through gravitational collapse. This video illustrates the temporal evolution of the formation process and its growth.
© Adnan-Ali AHMAD - CRAL