Planet formation and disc dynamics in stellar clusters and misaligned binary systems

Abstract

In this thesis I investigate the formation and dynamics of planets and protoplanetary discs, subject to external gravitational perturbations. In the first project, I study numerically the response of an embedded gapopening planet in a protoplanetary disc to the gravitational potential of a secondary star on a parabolic, coplanar orbit. I find that the perturbation by the secondary can cause substantial compactification and structural changes of the disc, resulting in modification of the inner and outer Lindblad torques and leading to outward migration of the planet. Hence this scenario provides a mechanism for stopping or slowing the inward migration of gap forming planets. In a second project I investigate the response of a gaseous disc to the presence of a binary companion, whose orbital plane is misaligned with respect to the disc midplane. I examine the resulting disc structure as a function of disc thickness and viscosity. For thick discs with low viscosity I find that the disc precesses as a rigid body with a negligible twist and warp. For thin discs whose viscosity is large, I find that they become highly twisted due to differential precession, but eventually attain a rigidly precessing state in which the twist is a smoothly varying function of radius. In a third project I introduce planetesimals into a misaligned binary system and study their collisional velocities to estimate whether collisions will lead to accretion or erosion. I generally find that collisional velocities tend to be higher than in coplanar simulations, due to misalignment of their orbital planes. I suggest that planetesimals cannot grow by collisions, unless sizes of 10km have already formed by another process. If the inclination becomes too high, the Kozai effect will lead to very large collisional velocities, and formation of planets should be impossible