AU Mic exhibits one of the most surprising and least understood behavior in debris disks - it appears to be emitting fast-moving dust clouds toward just one side of the disk. These fast-moving features were discovered by the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) team (Boccaletti et al. 2015). Using clues from the observations, we trace back in time and speculate that about 10,000 years ago, a 100 km sized object was destroyed in a catastrophic collision. The debris from the destruction spread in to a thin ring that continuously collides with the primary dust ring around AU Mic, producing small, sub-micron size particles that are blown away in AU Mic's energetic solar winds. These grains multiply as they travel through the primary dust ring, in a way similar to avalanches. Together with a model for the variability in AU Mic's solar activity, we were able to reproduce the appearance, brightness, and kinematics of the dust clouds.

     Super-Earths, planets of 2-20 Earth masses, make up the bulk of all known exoplanets. If we were to place them in circumstellar disks, where they, presumably, were formed, something contradictory happens - they would start migrating at an alarming rate due to their gravitational interaction with the disk, and so super-Earths should not be found where they are now! These planets can be saved if we additionally consider how their gravity would in turn affect the disk, and treat the disk gas as inviscid. Planets repel disk gas away from their orbits, so that as they migrate, gas accumulates ahead of the planets, and the feedback effect stalls and even stops the migration. Over million-year timescales, multiple super-Earths can torque most of the gas out of interplanetary space, either inward to feed their stars, or outward to be blown away in a wind, hollowing the inner regions of disks. We combined this theory with pebble and gas accretion models, and find that it naturally explains why many super-Earths are found at sub-au distances, while gas giants are typically located beyond about 1 au.

     How does gas flow around a small, embedded planet? Pushing PEnGUIn's capability to its limit, we performed high-resolution 3D simulations capturing both the global flow field of the disk and the atmosphere of the planet simultaneously. We identified the transient horseshoe flow which results in an open horseshoe region that constant exchanges material with the disk. We found fast, near-sonic flow within the planet's Bondi sphere which prevents the planet from gathering a hydrostatic atmoshphere. On the left is the streamlines along the widest horseshoe orbits. Click to see it in 3D (87MB)!

     Jupiter-size planets are capable of clearing out material near their orbits and opening gaps in disks. On the other hand, it was previously unknown how clean these gaps are. How massive does a planet need to be in order to open an observable gap? This turns out to be closely related to disk properties as well, such as temperature and viscosity. Using PEnGUIn simulations and verified by independent ZEUS90 simulations, we studied the parameter space and derived scaling laws for how empty gaps are.

    Irradiation Instability (IRI) is a form of disk instability caused by the combined effects of radiation pressure and light extinction. It can potentially operate on highly-irradiated disks, such as transitional disks around T Tauri stars and decretion disks around Be and Oe stars, creating temporal variability in their emissions. On the left is a snapshot of a IRI mode developing on the inner edge of a disk. Click the picture to see the full video (25MB).