On the Orbital Evolution of Giant Planet Pair Embedded in a Gaseous Disk. III. simultaneous gas accretion and the final mass ratio

Abstract

Recent exoplanet surveys reveal a systematic trend in the mass ratios of giant planet pairs ($> 30 M_\oplus$). Although the $M_{outer}/M_{inner}$ is broadly centered near unity, it exhibits a dependence on orbital separation. Compact pairs tend to have comparable masses, whereas widely separated pairs show outer-heavy configurations. To investigate the origin of this trend, we perform two-dimensional hydrodynamic simulations that couple gas accretion and orbital migration of two giant planet embryos with initial masses of $15-30 M_\oplus$. We explore the effects of disk viscosity, surface density slope, and initial mass ratio on the coupled evolution of planetary growth and migration. We find that convergent migration drives the planets to open a common gap, restructuring the gas flow. This configuration induces asymmetric accretion, in which the inner planet becomes starved as gap clearing suppresses its gas supply, while the outer planet continues to accrete from the high-density region at the outer gap edge. Consequently, the system naturally evolves toward mass-ratio convergence, and in many cases produces an inversion of the initial core-mass hierarchy. We further show that migration outcomes are highly sensitive to initial conditions. Systems with initially more massive inner cores tend to diverge, whereas steeper surface density profiles and more massive outer cores favor convergent migration and subsequent resonance capture. Overall, our results identify common-gap-driven asymmetric accretion as a unified mechanism linking orbital architecture, resonant capture, and the observed mass-ratio demographics of giant planet pairs.