Dynamics and Growth of Dust Grains in the Protoplanetary Nebula

Abstract

In the standard scenario of planet formation, the planetesimals grow from primordial dust grains of interstellar origin embedded in a gas disk, the so-called protoplanetary nebula. Large-scale gaseous structures, such as spiral waves and vortices, are likely to survive for a large number of rotation periods and can strongly influence the dynamics and growth of the dust grains. This has been stressed for the first time by P. Barge & J. Sommeria (1995, A&A, 295, L1) in the case of the vortices with a new scenario for the formation of the planetesimals and the rapid growth of the giant planet cores. This dissertation is concerned with the action of the mean flow of the gas on the dynamics of the solid dust particles. To reach this goal, we implemented a direct time-dependent integration code that models the particle dynamics under gravitational and viscous forces in a zero-thickness disk. A parallel version of this highorder accuracy particle-mesh code has been performed to carry out the potential role of large-scale vortices and spiral perturbations in the protoplanetary disks. The results show that the particles do not drift inward to the star as occurs in a standard symmetrical nebula. On the one hand, vortices tend to capture a large number of the particles. The effectiveness of this size-selective concentration mechanism depends on the value of the drag coefficient and on the distance from the Sun but also on the elongation of the vortex and its characteristic lifetime. We also found analytical expressions for the capture time as well as capture constraints as a function of the friction parameter, the elongation of the vortex, and the impact parameter. On the other hand, in the case of spiral perturbations, the velocity field can be described by a set of concentric streamlines as is usual in the kinematical description of a spiral wave. Moreover, since protoplanetary disks are nearly Keplerian, the spiral mode is likely the most probable and the most m p 1 robust one (E. Lee & J. Goodman 1999, MNRAS, 308, 684). In such a model, the streamlines are confocal ellipses and the particle trajectories become nearly coincident with them. With such a velocity field, we find that the particle trajectories end up in stable elliptic orbits and that the resulting dust surface density is significantly increased in the vicinity of the spiral pattern, incidentally facilitating the growth of the particles. We find also that a ringlike swarm of particles, with a given size distribution, tends to spread into a spiral-like pattern. This pattern results from the size segregation of the particles under the friction drag force. In summary, our results show that small disk asymmetries, due to giant vortices or spiral patterns, are able to confine the solid material within some small regions of the protoplanetary nebula. By increasing the lifetime and the surface density of the solid particles, these confining mechanisms offer new possibilities for the formation of the planetesimals and the giant planet cores and could help also in explaining a rapid formation of the extrasolar giant planets.