Speaker
Description
Planetesimal formation in protoplanetary disks is still one of the major open questions in planet formation theory. It is known that solids can’t grow up to asteroid size relying on sticking after pairwise collisions only, due to the fragmentation barrier and the drift barrier. A possible solution is to form dense particle clumps, with low velocity-dispersion, that can then collapse under self-gravity. Streaming instability is the most popular mechanism for concentrating dust particles and it can be seen as a turbulent mechanism. Turbulence in disks is then critical for planetesimal formation. In this context, we want to study the dynamics of particles in turbulent flows with Keplerian rotation and shear. To treat this astrophysical problem we use fluid-dynamics methods, trying to provide innovative perspectives on this challenging question. We perform 2D direct numerical simulations using the shearing box approach and we explore various values of the rotation frequency Ω and the solid stopping time ts, a parameter related to the particle size. We then analyse the results using tools borrowed from the study of dynamical systems. In particular, the Lyapunov dimension dL is calculated for each run to characterize the dust dynamics in the flow. This quantity gives an estimation of the fractal attractor dimension in the phase space. We find three different regimes. For low values of Ω and large values of ts we obtain dL > 2, therefore the inertial particles fill the whole space. Focusing instead on intermediate values of ts, for small rotation rates the particles are expelled from the eddies and form fractal structures, while they tend to concentrate inside the anticyclones for larger Ω. Particles eventually form a pointwise cluster for dL = 0. We have identified promising tools for the understanding of planetesimal formation.
