Formation of 51 Pegasi-Type Systems
T. V. Ruzmaikina
Lunar and Planetary Laboratory, University of Arizona, Tucson AZ
The discovery of giant Jupiter-type planets in a close proximity to 51 Peg, Tau Boo, and Upsilon And has prompted the question of where the planets could be formed in the circumstellar disks of solar-type stars. It has been suggested that the planet around 51 Peg was formed at several AU, where the phase transition between water vapor and ice occurs, and then it migrated toward the star, loosing its angular momentum because of tidal interaction with a remnant circumstellar disk (Boss 1995; Lin 1995).
However, there are no evident physical reasons to exclude the possibility of a giant planet forming closer to the star. The giant planet can be formed either due to gravitational instability in the gaseous component of circumstellar disk, or through the process of the accumulation of solid core, followed by accretion of the gaseous atmosphere. One can not exclude also that these Jupiter-mass planets are composed of rocky materials and accumulated as solid bodies.
Planet formation, at distance of ~0.05 AU from the star, requires that the temperature of the circumstellar protoplanetary disk is low enough for the condensation of rocky materials, and the surface density is sufficiently high. The gravitational instability imposes more strict restrictions on the parameters of the disks.
In this paper, I construct a model of the formation/evolution of protoplanetary disks which can result in the formation of a massive planet at distances of the discovered planets. The models are characterized by relatively low angular momenta of the protosolar clouds, and low rate of viscous accretion.
An additional help for planet formation could come from the tentative enrichment in solids (by a mechanism similar to that suggested earlier by Stevenson and Lunine) of the region where the disk temperature is slightly lower than condensation temperatures of the abundant rocky materials and metals.