Enhancement of Core Accretion by an Extended Low-Mass Envelope

The early stages of the formation of Jupiter are modeled via core nucleated accretion and gas capture. The core is initially a seed body with a radius of 350 kilometers, i.e., 1e-4 Earth masses (Me), and orbits in a disk of planetesimals whose initial size distribution ranges from ~10 meters to 100 kilometers. The size distribution of solids evolves through growth and fragmentation of planetesimals, whose orbits are affected by viscous and gravitational stirring, velocity damping, and drag-assisted migration. The seed body has an orbital semi-major axis of 5.2 AU and the initial surface density of solids at that distance is 10 grams per square centimeters. The mass growth of the core is initially fast, reaching 1 Me in about 7e4 years, but the core does not grow larger than about 4 Me in ~1 Myr if the accretion of solids is determined by the geometrical cross-section of the core. The formation of a gaseous envelope via gas capture by the core substantially enhances the size-dependent cross-section of the planet for accretion of planetesimals. The calculation of the envelope structure includes a self-consistent treatment for dust opacity, which takes inot account coagulation and sedimentation of dust grains released in the envelope as passing planetesimals are ablated. The envelope-enhanced accretion rate of solids results in a core mass of about 7 Me after about 0.5 Myr, when the envelope mass is approximately 0.3 Me, at which point the accretion rate of gas surpasses that of solids. Support from NASA Outer Planets Research Program is gratefully acknowledged.