Evolution of migrating planets undergoing gas accretion

We analyze the orbital and mass evolution of planets that undergo runaway gas accretion by means of two- and three-dimensional hydrodynamic simulations. The disk torque distribution per unit disk mass as a function of radius provides an important diagnostic for the nature of the disk-planet interactions. We present results of simulations for mass-gaining, migrating planets. For planets with an initial mass of 5 M-circle plus, which are embedded in disks with standard parameters and which undergo runaway gas accretion to 1 M-J, the torque distributions per unit disk mass are largely unaffected by migration and accretion for a given planet mass. The migration rates for these planets are in agreement with the predictions of the standard theory for planet migration (type I and type II migration). The planet mass growth occurs through gas capture within the planet's Bondi radius at lower planet masses, the Hill radius at intermediate planet masses, and through reduced accretion at higher planet masses due to gap formation. During runaway mass growth, a planet migrates inward by only about 20% in radius before achieving a mass of similar to 1 M-J. For the above models, we find no evidence of fast migration driven by coorbital torques, known as type III migration. We do find evidence of type III migration for a fixed-mass planet of Saturn's mass that is immersed in a cold and massive disk. In this case the planet migration is assumed to begin before gap formation completes. The migration is understood through a model in which the torque is due to an asymmetry in density between trapped gas on the leading side of the planet and ambient gas on the trailing side of the planet.