ASTROPHYSICS

Scientists of the Max Planck Institute for Astronomy in Heidelberg are using the HPC infrastructure of the Jülich Supercomputing Centre for extensive magneto-hydro-dynamical and million particle simulations of protoplanetary disks to study their evolution and properties. Findings are helping the researchers to understand the processes leading to the formation of planets, moons and asteroids. Their investigations will help to explain the observed diversity in planetary systems and in our own solar system.

Planet formation is a beneficial side effect of star formation. When a large gas cloud collapses under its own weight creating a star, some of the material forms a disk out of dust and gas around this newborn star. Such disks can be found around stars younger than 10 million years, and we believe they are the origin of planetary systems like ours. They are composed mainly out of Hydrogen and Helium, plus 2% heavier elements which Astronomers simply call metals. The initial metal abundance is 13 orders of magnitude lower than typical planet densities. Thus, the planet formation process must be very efficient in concentrating metals locally.

The first step towards planet formation is the coagulation of dusty ice grains. By sticking, these grains grow up to centimetres in size. Larger sizes are not possible since collisions then rather lead to destruction than to sticking. What follows is unfortunately not observable because metre-sized or even kilometre-sized objects are not detectable. Yet, having discovered thousands of planets around other stars the planet formation process must be very efficient in overcoming this and other growth barriers.

From our own solar system we know that planetesimals must have been formed, which are planetary building bricks of 100 kilometres in size. Those then formed cores of gas giants and terrestrial planets. Planetesimal leftovers can still be found today in the form of asteroids and comets. Thus, we would have a relatively clear evolutionary path from dust up to the final planetary system if we could explain how centimetre-sized pebbles can grow to planetesimals. 

Our simulations use the Pencil Code [1] which contains a high order finite difference MHD code. Particles are treated as Lagrangian swarm particles. Our runs use up to 5,123 grid-cells and 64 million particles [2]. They run for millions of computational steps before planetesimals start to form because the concentrations develop on viscous time scales, which are much longer than the dynamical time scales.

Currently we push our simulations to understand the efficiency of this process and to derive an initial mass function for planetesimals. The first goal will be achieved by better modelling the turbulence in disks and the second by performing high-resolution studies of the collapsing and possibly fragmenting particle clouds. With both results in hand we can predict when and where planetesimals should form and of which size they will be. This is a fundamental step forward in understanding the formation of our own solar system as well as of the many planetary systems around other stars.

Acknowledgements:
This project has received funding from the Deutsche Forschungsgemeinschaft within the Schwerpunktprogramm (DFG SPP) 1385 "The first ten million years of the solar system". The authors gratefully acknowledge the Gauss Centre for Supercomputing (GCS) for providing computing time for a GCS Large Scale Project on the GCS share of the supercomputer JUQUEEN at Jülich Supercomputing Centre (JSC).

References:
[1] http://pencil-code.nordita.org/
[2] Johansen, A., Klahr, H., Henning, Th. Astronomy and Astrophysics, 529, 62, 2011

PD. Dr. H. Hubertus Klahr 
Max-Planck-Institut für Astronomie 
Königstuhl 17, D-69117 Heidelberg, Germany
e-mail: klahr@mpia.de
http://www.mpia.de/homes/klahr