Origin of complex non-axisymmetric structures in Type 2 Transition Disks

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The enormous resolving power and sensitivity of the Atacama Large Millimeter Array has revealed what protoplanetary disks look like at high angular resolution. A picture is emerging that most disks are to a large extent axially symmetric, and generally have strong radial (multi-ringlike) structure. The origin of these rings is not yet understood, but it is also not quite unexpected: due to the differentially rotating nature of Kepler's law, any inhomogeneity in the disk is bound to be azimuthally smeared out into a ring. Moreover, any planet present in the disk will tend to open an annular gap in the disk. So the real surprise is that in the current high resolution surveys with ALMA about 5-15\% of the disks show non-axisymmetric features. These include $m=2$ spirals, strange dusty arcs, as well as disk warps where the inner disk is inclined with respect to the outer disk. These non-axially symmetric features must be related to forces that are strong enough that they can compete with the ``smearing out'' nature of differential keplerian rotation. In other words: These features are an indication that something is going on in these disks that goes beyond the standard protoplanetary disk physics. This could be the presence of a young planet, the presence of a dust-trapping vortex, the action of an instability, or a number of other processes. Learning about them is a unique chance at gathering observational insight into the process of planet formation. The goal of this project is to test a range of scenarios, including those that are rooted in planet formation, for the explanation of the observed non-axisymmetric features, and to decypher the message that is hidden in these features and hence improve our understanding of planet formation.