Models of the in situ formation of detected extrasolar giant planets

We present numerical simulations of the formation of the planetary companio ns to 47 UMa, rho CrB, and 51 Peg. They are assumed to have formed in situ according to the basic model that a core formed first by accretion of solid particles, then later it captured substantial amounts of gas from the prot oplanetary disk. In most of the calculations we prescribe a constant accret ion rate for the solid core. The evolution of the gaseous envelope is calcu lated according to the following assumptions: (1) it is in quasi-hydrostati c equilibrium, (2) the gas accretion rate is determined by the requirement that the outer radius of the planet is the place at which the thermal veloc ity of the gas allows it to reach the boundary of the planet's Hill sphere, (3) the gas accretion rate is limited, moreover, by the prescribed maximum rate at which the nebula can supply the gas, and (4) the growth of the pla net stops once it obtains approximately the minimum mass determined from ra dial velocity measurements (in one case the planet is allowed to grow to tw ice this limit). Calculations are carried out through an initial phase duri ng which solid accretion dominates, past the point of crossover when the ma sses of solid and gaseous material are equal, through the phase of rapid ga s accretion, and into the final phase of contraction and cooling at constan t mass. Alternative calculations are presented for the case of 47 UMa in wh ich the solid accretion rate is calculated, not assumed, and the dissolutio n of planetesimals within the gaseous envelope is considered. In all cases there is a short phase of high luminosity (10(-3)-10(-2) L.) associated wit h rapid gas accretion. The height and duration of this peak depend on uncer tain model parameters. The conclusion is reached that in situ formation of all of these companions is possible under some conditions. However, it is m ore likely that orbital migration was an important component of the evoluti on, at least for the planets around rho CrB and 51 Peg. (C) 2000 Academic P ress.