Models of Accreting Gas Giant Protoplanets in Protostellar Disks
  (Abridged) We consider models of gas giant planets forming in protoplanetary disks consisting of solid cores with gaseous envelopes in contact with their critical Hill spheres while accreting gas from the surrounding disk.We suppose the luminosity derives from gas accretion alone.We label such models as type A and follow their evolution which may occur on a time scale similar to the protostellar disk lifetime until rapid gas accretion. We consider another set of models, we label type B, with a free surface, powered by gravitational contraction, while accreting through a disk.We find these models rapidly attain a radius <~ 2x10^(10)cm without subsequent expansion.We speculate that giant planet formation is initially described by models of type A, until at the onset of rapid gas accretion, there is a transition to models of type B. Protoplanet migration in standard models tends to be most effective near this transition where it also changes from type I to type II.If a mechanism prevents type I migration of low mass protoplanets, a rapid inward migration might occur near the transitional mass regime. Such protoplanets would end up in the inner disk regions undergoing type II migration and further accretion potentially becoming sub Jovian close orbiting planets. Noting that dustier more massive cores spend longer at a larger transitional mass where faster migration is expected, these may be more prone to end in close orbiters.We find the luminosity of the protoplanets during the later stages is dominated by the circumplanetary disk and protoplanet disk boundary layer.For one Jupiter mass the luminosity range is 10^-(1.5-4) L_sun$ depending on the evolutionary stage and external conditions. 
John C.B. Papaloizou and Richard P. Nelson