[Paper Review] PPV Chapter - The Formation of Brown Dwarfs

A. P. Whitworth, Matthew R. Bate+3 more|arXiv (Cornell University)|Feb 16, 2006

Astro and Planetary Science1 references3 citations

TL;DR

This paper reviews five formation mechanisms for brown dwarfs—turbulent fragmentation, core collapse and fragmentation, disc fragmentation, dynamical ejection, and photo-erosion—concluding that they form like low-mass stars via gravitational instability on dynamical timescales, with their minimum mass constrained to 0.001–0.004 M☉ under contemporary conditions. The mechanisms are not mutually exclusive, and their relative importance depends on environment, with full understanding requiring advanced 3D radiative MHD simulations to resolve angular momentum, radiation transport, and magnetic fields.

ABSTRACT

We review five mechanisms for forming brown dwarfs: (i) turbulent fragmentation of molecular clouds, producing very low-mass prestellar cores by shock compression; (ii) collapse and fragmentation of more massive prestellar cores; (iii) disc fragmentation; (iv) premature ejection of protostellar embryos from their natal cores; and (v) photo-erosion of pre-existing cores overrun by HII regions. These mechanisms are not mutually exclusive. Their relative importance probably depends on environment, and should be judged by their ability to reproduce the brown-dwarf IMF, the distribution and kinematics of newly formed brown dwarfs, the binary statistics of brown dwarfs, the ability of brown dwarfs to retain discs, and hence their ability to sustain accretion and outflows. This will require more sophisticated numerical modelling than is presently possible, in particular more realistic initial conditions and more realistic treatments of radiation transport, angular momentum transport and magnetic fields. We discuss the minimum mass for brown dwarfs, and how brown dwarfs should be distinguished from planets.

Motivation & Objective

• To determine whether brown dwarfs form via the same mechanisms as low-mass H-burning stars, given their continuous initial mass function (IMF) and kinematic properties.
• To evaluate the minimum mass for brown dwarf formation across different astrophysical scenarios, including turbulent fragmentation and disc instability.
• To assess the role of competing physical processes—such as competitive accretion, dynamical ejection, and photo-erosion—in shaping the observed brown dwarf population.
• To distinguish brown dwarfs from planets based on formation timescale and initial elemental composition, arguing for a fundamental divide between stellar and planetary formation pathways.
• To identify the limitations of current simulations and the need for more realistic, 3D, radiative, magneto-hydrodynamical models to resolve the physics of fragmentation, angular momentum transport, and radiation effects.

Proposed method

• Analytical derivation of the minimum mass for opacity-limited fragmentation in three scenarios: hierarchical 3D fragmentation, 2D shock-compressed layer fragmentation, and Toomre-unstable disc fragmentation.
• Numerical simulations of star cluster formation in large protocluster cores to model concurrent processes: core collapse and fragmentation, disc fragmentation, competitive accretion, and dynamical ejection.
• Use of N-body dynamics to model protostellar embryo interactions, including ejection events that terminate accretion and cap masses.
• Incorporation of radiative transfer effects in simulations to assess their impact on disc fragmentation and the formation of close brown dwarf binaries.
• Comparison of simulated brown dwarf properties—such as mass function, binary statistics, disc retention, and kinematics—with observational data.
• Assessment of disc size and survival in different core environments (high vs. low turbulence), using simulation outputs to infer disc size dependence on birth conditions.

Experimental results

Research questions

• RQ1 Do brown dwarfs form via the same dynamical, gravitational instability processes as low-mass H-burning stars, or are they fundamentally different in origin?
• RQ2 What is the minimum mass for a prestellar core to fragment into a brown dwarf, and how does this depend on the formation scenario (e.g., turbulent vs. disc fragmentation)?
• RQ3 How do competing processes—such as competitive accretion, dynamical ejection, and photo-erosion—shape the observed mass function, binary statistics, and kinematics of brown dwarfs?
• RQ4 To what extent do disc fragmentation and disc survival influence the formation of close brown dwarf binaries and the retention of circumstellar discs?
• RQ5 What physical conditions (e.g., turbulence, radiation, magnetic fields) determine whether a protostellar embryo becomes a brown dwarf or a low-mass star?

Key findings

• The minimum mass for brown dwarf formation via opacity-limited fragmentation is estimated at 0.001–0.004 M☉ in the solar neighborhood, depending on the formation scenario.
• Brown dwarfs formed in highly turbulent cores are predicted to have small discs (R < 10 AU), while those from low-turbulence cores may retain larger discs (R ≥ 10 AU), suggesting disc size depends on birth environment.
• In simulations, only one dynamically ejected brown dwarf retained a resolved disc (R ≥ 10 AU), indicating that ejection events likely truncate disc growth.
• Simulated brown dwarf–brown dwarf binaries have separations < 20 AU, but resolution limits prevent full validation, suggesting missing physics (e.g., radiative transfer) may affect outcomes.
• Disc fragmentation at large radii (≥ 100 AU) is favored by thermodynamic conditions such as the H₂ dissociation and opacity gap at ~2000 K, promoting formation of close binaries.
• All five formation mechanisms—turbulent fragmentation, core collapse, disc fragmentation, ejection, and photo-erosion—operate concurrently in advanced cluster simulations, with their relative contributions likely varying by environment and epoch.