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Unveiling Planet Formation by Observations and Simulations

Periodic Reporting for period 2 - UFOS (Unveiling Planet Formation by Observations and Simulations)

Reporting period: 2019-12-01 to 2021-05-31

The goal of the UFOS project is to unveil planet formation, using state of the art simulations and observations. We want to understand the gas and dust dynamics and evolution in protoplanetary disks. Where are the pebbles concentrated in disks? Where can planets form? What is the influence of the dust to the gas and vice versa. These questions we want to address with UFOS using state-of-the-art theoretical models. These models allow us to solve for the hydrodynamical and magneto-hydrodynamical equations for a full global disk model. The results show us the detailed dynamics of the gas, similar as it was for our own solar system 4.5 billion years ago. To understand how the planets are formed is one of the biggest questions in modern astrophysics.
We have worked and published many results ranging from the hot innermost regions of protoplanetary disks until the cold outer disk regions. We have investigated the conditions for planet formation at the dust sublimation zone. Dust grains which grow in the disk, drift to the inner disk regions where there get trapped due to a pressure maximum. Our results show that there is a robust pressure maximum due to the transition of accretion stress caused by the ionization transition. Here the magneto-rotational instability is getting damped which leads to a rise in the surface density. Our results also show that only under certain conditions we expect the grains to get trapped at the inner rim. In case the turbulence is too high we found that the grains do not grow that fast and can pass through the pressure trap. We also found that young forming proto-planets can get trapped at the inner rim which could lead to the potential formation of a chain of super-Earth, similar as it was found for the Trappist-1 system. We have also worked on the gas and dust dynamics in the so called dead-zone. Here we expect hydrodynamical instabilities to dominate. With our works we have investigated the strength of the vertical shear instability (VSI). We found that the instability can efficiently mix up the grains in the disk. We also found in high-resolution simulations that the gas becomes turbulent in between the large scale motions of the vertical shear instability. Further we have found that there is also a accretion stress transition zone for the VSI which causes the pebble drift to slow done. This could also generate gaps and rings, which are currently observe with radio interferometer telescopes. We have developed a new dust method to solve for the individual motions of dust pebbles. We are also collaboration with several other teams to understand for the scattered light and dust continuum observations of several protoplanetary disks.
We have also made progress beyond what we expected. This includes the characterisation of dust grains, their properties to absorb and scatter radiation. Our result showed that the detailed shape of the grains is very important for the efficiency to scatter and polarise the radiation. Recent state-of-the-art observations showed the dust continuum radiation is polarised which could be caused either by the intrinsic alignment of magnetic fields or by self-scattering. For both processes the detailed characteristics how grains absorb, scattered and change the polarisation vector is of highest importance, even more when the grains are porous. Especially when grains are small we do not expect that the grains are very compact objects.