PhD Defense by Michael Küffmeier – Niels Bohr Institute - University of Copenhagen

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PhD Defense by Michael Küffmeier

Accretion Processes in Star Formation

--- Accounting for different environments



Stars and their corresponding protoplanetary disks form in different environments of Giant Molecular Clouds. By carrying state-of-the art zoom-simulations with the magnetohydrodynamical code ramses, I investigated the accretion process around young stars that are embedded in such different environments. Starting from a turbulent (40 pc) 3 Giant Molecular cloud, efficient use of Adaptive Mesh Refinement  technique allowed to resolve the processes inside of protoplanetary disks with grid sizes down to 0.06 AU, thus covering a range of spatial scales of more than six orders of magnitude. Accounting for short-lived radionuclides that enrich the cloud as a result of supernova explosions of the massive stars allows us to analyze the distribution of the short-lived radionuclides around young forming stars.

In contradiction to results from highly-idealized models, we find that the discrepancy in 26 Al abundance in different types of the oldest solids of the Solar System (Calcium Aluminum rich inclusions) is not a result of early supernova injections. Instead, our results suggest thermal processing of dust grains as a likely scenario for the measured differences. Furthermore, the simulations show that the accretion process of stars is heterogeneous in space, time and among different protostars. In some cases, disks form a few thousand years after stellar birth, whereas in other cases disk formation is suppressed due to efficient removal of angular momentum. Angular momentum is mainly transported outward in the radial direction, due to the structure of the large scale magnetic field, and the results suggest that disks form because the turbulence reduces the strength of magnetic braking. A further analysis of the accretion process of the different stars shows that the presence of the disks correlates with potentially observable fluctuations in the luminosity profile that are induced by variations in the accretion rate. Considering that gas inside protoplanetary  disks is not fully ionized, I implemented a solver that accounts for nonideal MHD effects into a newly developed code framework called dispatch. Considering the increasing number of indications for early planet formation, this work provides important constraints for modeling the formation of planets in young protoplanetary disks.

Supervisor: Åke Nordlund