10 August 2019


Leonardo Krapp

A thesis for the degree of Doctor of Philosophy defended August 2019.

The PhD School of Science, Faculty of Science, NBIA, Niels Bohr Institute, University of Copenhagen

Oliver Lothar Gressel

Multispecies Dynamics in Weakly Ionized Dusty Protoplanetary Disks

 The formation of the Solar System remains as one of the major mysteries in the modern Astrophysics. Planet Formation is believed to be a consequence of the formation of a star, where the rotating cloud surrounding the protostar collapses into a disk giving birth to a protoplanetary disk consisting of partially ionized gas and dust. In recent years, ALMA provided the first high resolutions observations of protoplanetary disks unveiled the presence of very rich substructures such as spiral arms, rings, and gaps. However, it is not clear yet whether the observed features are signatures of planets in formation, given the complexity of the astrophysical processes taking place at each of the different disk scales. In particular, collisions between the multiple species that compose the disk mixture play an important role in affecting different processes related to accretion mechanisms, the growth of dust particles, and the formation of planetary embryos. Thus, in this thesis, we focus our efforts on studying the momentum transfer between different species and its effect on the evolution of the disk.  Despite disks being poorly ionized, charged species can transfer energy and momentum from the magnetic field to the neutrals due to drag forces. Besides, the aerodynamical coupling between dust particles and the gas has significant consequences for the dust dynamics and evolution. In this thesis, we introduce a framework to solve the collisions between multiple species with particular emphasis on dust dynamics. We show that our scheme, and its implementation in the code FARFO3D, is suitable, and very robust, to study the self-consistent dynamics of several fluids. We study the formation of large-scale vortices at the mid-plane of the disk induced by the non-linear interaction between the gaseous component and the magnetic field. For this, we assume a weakly ionized regime where electrons can move freely, and ions are well-coupled to the neutrals, so-called Hall regime in the context of protoplanetary disks. We furthermore assess the effect of these vortices on the dust evolution, and we demonstrate that the emerging features are capable of accumulating dust grains. In addition, we present a discussion on the linear and non-linear phase of the streaming instability. Such instability is believed to play a vital role in the formation of planetary bodies and relies on the local concentrations of dust particles that experiment a drift with respect to the gas. We present the first systematic study of the multispecies linear growth of this instability characterizing the dust component via a particle-size distribution and show that the multispecies instability may growth on timescales much longer than the case with only one dust sizes, sometimes comparable to the disk lifetime. 

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