Radiation Hydrodynamics of Self-gravitating Protoplanetary Disks. I. Direct Formation of Gas Giants via Disk Fragmentation

Abstract Gravitational instability (GI) has long been considered a viable pathway for giant planet formation in protoplanetary disks (PPDs), especially at wide orbital separations or around low-mass stars where core accretion faces significant challenges. However, a primary drawback is that disk fragmentation from GI is generally found to produce overmassive clumps, typically in the mass range of brown dwarfs, although most numerical studies adopt simplified cooling prescriptions or those with limited numerical resolution. We conduct a suite of global three-dimensional radiation hydrodynamics simulations of self-gravitating PPDs using the meshless finite-mass method. By implementing radiation transport via the M1 closure and systematically varying disk mass and opacity, we show that increasing disk mass and lowering opacity promote fragmentation by enhancing radiative cooling. Nonfragmenting disks settle into a gravitoturbulent state with low-order spiral structures and effective angular momentum transport characterized by <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>α</mml:mi> <mml:mo>∼</mml:mo> <mml:msubsup> <mml:mi>β</mml:mi> <mml:mi>cool</mml:mi> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . In fragmenting disks, a subset of gravitationally bound clumps survives as long-lived fragments. Their initial masses form a consistent distribution around Σ · λ T · 2( c s /Ω K ) (with λ T denoting the Toomre wavelength), corresponding to ∼0.3–10 M J in our simulations, consistent with the masses of gas giants. These results demonstrate that GI can produce planet-mass fragments under more realistic conditions, reinforcing it as a viable gas giant formation pathway and motivating further studies of fragment evolution and observational signatures.