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Physics - Overview

Research Interests

			Planet formation and planetesimal formation
			Smoothed Particle Hydrodynamics (SPH)
			Simulation of solid and porous media
			Collision simulations

Current status

At the beginning of July I handed in my PhD thesis in Computational Astrophysics about "Simulation of Pre-Planetesimal Collisions with Smoothed Particle Hydrodynamics". This project is funded by the DFG Research Unit 759 "Formation of Planets" and supervised by PD Dr. Roland Speith.

Description

The project addresses the field of planet formation. In the standard scenario, planets form in protoplanetary discs around young host stars. The protoplanetary discs consist mainly of gas and a small fraction of dust, which initially exists as a population of micron sized dust grains. Two important hypothesis are put forward to explain planet formation in these discs. The gravitational instability hypothesis assumes that overdense subregions of the disc become gravitationally unstable and collapse into objects of kilometre or even dwarf planet size. The coagulation hypothesis suggests that planets form by successive collisions starting from dust grains.



In the framework of the coagulation scenario, interactions between gas and dust aggregates in the disc induce relative velocities. Planet formation proceeds on three stages: the fractal stadium, the pre-planetesimal stadium, and the planetesimal stadium. In the fractal stadium, the relative velocities are low and dust aggregates collide and stick due to van der Waals forces. The mass of the aggregates is related to their radius by a power < 3, i.e., the growth is fractal. By this process millimetre- to centimetre-sized porous dust aggregates form. In the pre-planetesimal stadium, collision velocities increase and growth is endangered by disruptive events. The sticking mechanism in this regime is unclear. Once by some mechanism a sufficient population of kilometre-sized planetesimals was formed, growth proceeds to planets in a runaway fashion by gravity-assisted accretion.


My research project addresses the pre-planetesimal stadium. The investigation of pre-planetesimal collisions by laboratory techniques is size limited. This is because experiments with fragile aggregates are infeasible beyond centimetre size in microgravity and vacuum. For this reason within the project a numerical porosity model was developed and calibrated for the simulation of pre-planetesimal dust material by means of laboratory benchmark experiments. At the end of the calibration process compaction, bouncing, and fragmentation behaviour of dust aggregates could be simulated quantitatively correct. The numerical model is now applied for an in-depth investigation of two-body pre-planetesimal collisions with respect to collision parameters such as impact velocity and impact parameter, object porosity, size ratio and rotation.

Pre-planetesimal collision sequence with impact parameter (SPH simulation).

Detailed statistics about pre-planetesimal collisions are required as input parameters for global dust coagulation models. These compute the evolution of dust aggregates in protoplanetary discs taking into account the dust-gas interaction and the resulting collision velocities. To improve the information transfer between global coagulation models and numerical as well as laboratory dust collision results the four-population model was developed within this project. This classification scheme for outcomes of pre-planetesimal collisions by mass distinguishes the largest and second largest fragment, a fragment population, whose mass distribution can be modelled by a power-law, and a sub-resolution population which accounts for the limited resolution of a numerical method. The four-population model is accurate enough to capture any combination of sticking, bouncing, and fragmentation behaviour and simple enough to be implemented in global coagulation models.

The Demoreel of the B4 project visualises the calibration process as well as the most important simulation results. Other movies can be found in the YouTube Channel of the Computational Physics Group.

Selected publications

	2011		R.J. Geretshauser, R. Speith, W. Kley: "Collisions of inhomogeneous pre-planetesimals. Introducing an inhomogeneity damage model", Astronomy & Astrophysics, submitted
			R.J. Geretshauser, F. Meru, R. Speith, W. Kley: "The four-population model: a new classification scheme for pre-planetesimal collisions", Astronomy & Astrophysics, 531, A166 (DOI) (arXiv)
	2010		R.J. Geretshauser, R. Speith, C. Güttler, M. Krause, J. Blum: "Numerical Simulations of Highly Porous Dust Aggregates in the Low-Velocity Collision Regime. Implementation and Calibration of a Smooth Particle Hydrodynamics Code.", Astronomy & Astrophysics, 513, A58 (DOI)

For a complete list of publications consult the publications section.