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    Astrophysics

Text "Astrophysics"	Peter L. Biermann, Max Planck Institute for Radioastronomy, Bonn
The Origin of Planets	Hubert Klahr, Thomas Henning, Astrophysical Institute and University Observatory, and Wilhelm Kley, Theoretical Physics Institute, University of Jena
Radiation Hydrodynamical Simulations on the Evolution of Protostellar Disks	Sabine Richling, Harold W. Yorke, Astronomical Institute, University of Würzburg
Modelling a Molecular Jet	Roland Völker, Michael D. Smith, Harold W. Yorke, Astronomical Institute, University of Würzburg
Models of Globular Star Clusters	Marc Hemsendorf, Rainer Spurzem, Astronomisches Rechen-Institut Heidelberg
Chemodynamic Evolution of Dwarf Galaxies	Andreas Rieschick, Gerhard Hensler, Institute of Theoretical Physics and Astrophysics, University of Kiel

Text "Astrophysics"

Where we come from, where we are going to on earth, in the solar system, in our Milky Way and in our universe is described by physical laws. The universe produces higher energies, more extreme densities and has larger spaces and longer times than any earthly experiment.

Planets like the Earth originate from a gaseous disk around a young star; since the material in such a disk moves slowly towards the central star, this is called an accretion disk. If hot stars are present in the vicinity, they can destroy this disk in turn by their radiation and thus stop the growth of planet-like bodies. In many, perhaps in all, cases these disks with stars also have a gas jet which juts vertically outwards from the interior of the disk very close to the star. Such a disk-jet combination is also found in larger systems up to the relativistic flows in the gas jets which emanate from massive black holes.

Stars are frequently produced in groups and may form dense clusters, the globular star clusters. In our Milky Way, the globular clusters are witnesses to the initial phases of all star formation at the genesis of our galaxy. The evolution of closely packed star clusters is also a paradigm of the behaviour of the star clusters around massive black holes as in quasars, the most luminous phenomena in the universe known to us, which receive their energy from matter falling into the deep gravitational field around black holes.

But how galaxies like our Milky Way are formed can be understood best with the aid of small systems. All chemical elements from carbon onwards have their origin in the interior of massive stars. Most of these stars exploded during the early phases of our Milky Way. These stars then fly apart with an enormous energy (then called a supernova) and thus give us the oxygen for breathing, the silicon for our computers and the iron for our bridges, cars and railways.

But "whatever holds the world together in its inmost folds" (Goethe, Faust) is discovered best in particle collisions of the highest energies (such as, for example, in CERN, Fermilab, Stanford). The relativistic gas jets from the environment of massive black holes are probably extreme particle accelerators which can attain particle energies up to many powers of ten beyond those of earthly accelerators. They are experiments for us to investigate the interaction of high-energy particles made available to us by the cosmos, so to speak. But the most extreme energies, about which we can only speculate, are involved in the formation of our universe - and this can only be represented in detail by large computer simulations.

Our approach to a better understanding of the world has always been inspired by nature, which leads us to new concepts called theories having been developed mathematically, such as the "general theory of relativity".

But only quantitative predictions can be verified experimentally. We do not make experiments in the cosmos, but we can observe a multitude of often unexpected phenomena. For a detailed, quantitative and physical understanding of the origin of our Earth up to the interactions of the highest particle energies observed, large numerical simulations are frequently the only way to completely verify new physical concepts.

(Peter L. Biermann, Max Planck Institute for Radioastronomy, Bonn)

The Origin of Planets

Investigations concerning the physics of star formation and the associated origin of planets are a major subject of modern astrophysical research. Astronomical observations have led to the direct imaging of gas-dust disks resembling the presolar nebula from which planetary systems may evolve. In fact, the existence of such extrasolar planets has recently been proved. However, the resolving power of the observations is not high enough to obtain a detailed insight into disk physics. This is where numerical simulations may be helpful. They are to clarify how mass is transported inwards and angular momentum is transported outwards and how the evolution of micrometre-sized particles into planets proceeds. The aim of these simulations is to calculate the dynamic evolution of such disks. The picture shows the result of such a simulation with a grid of 64 x 40 x 128 points: convective velocity in the midplane of an accretion disk for a radial range between 4 and 6 astronomical units. Red regions designate rising and blue regions falling gas.

(Hubert Klahr, Thomas Henning, Astrophysical Institute and University Observatory, and Wilhelm Kley, Theoretical Physics Institute, University of Jena)

Radiation Hydrodynamical Simulations on the Evolution of Protostellar Disks

Young massive stars emit high-energy radiation and can destroy the circumstellar disks of neighbouring less massive stars. This process is investigated by radiation hydrodynamical simulations. A "numerical telescope" is used to calculate emission maps from the results, which then enable a direct comparison with observations. Such an emission map of a photoevaporating disk is displayed at radio wavelengths. The bright disk surface facing the radiation source and the emission along the tail are evident. This head-tail structure is typical of objects discovered by the Hubble Space Telescope in the Orion Nebula.

(Sabine Richling, Harold W. Yorke, Astronomical Institute, University of Würzburg)

Modelling a Molecular Jet

Emission map of a 3D model of a molecular jet - a strongly collimated flow of gas emitted by a young star. For simulation purposes, the underlying hydrodynamic and chemical equations were solved on a three-dimensional grid. The model data were then used to calculate the emission for a particular infrared emission line of molecular hydrogen. A focused beam of gas enters the integration region from the left and reacts with the gas present there. At the tip, a strong shock wave is formed in which the gas is heated and excited. The emission nodes inside the jet beam are generated by a periodic variation of the injection velocity.

(Roland Völker, Michael D. Smith, Harold W. Yorke, Astronomical Institute, University of Würzburg)

Models of Globular Star Clusters

The picture shows the projection of the spatial distribution of the stars of a model for a globular star cluster. The stars' colour depends on the speed at which they move through the cluster. Fast stars are characterized by a light yellow colour and slow ones by a dark red tone. Dynamical models of globular star clusters require a great deal of computer time due to the need for pairwise force calculation. This project attempts to find suitable criteria for applying this type of force calculation to just a small number of stars only. The force acting on the other stars is to be determined by a much more efficient field method.

(Marc Hemsendorf, Rainer Spurzem, Astronomisches Rechen-Institut Heidelberg)

Chemodynamic Evolution of Dwarf Galaxies

The evolution of rotating low-mass (masses of one billion suns) galaxies is modelled without (left column) and with (right column) the influence of a dark matter halo whose existence is postulated from the cosmological point of view and for reasons of galaxy kinematics. Since galaxies consist of gas of different temperatures and of embedded stars, which influence the gas and thus the evolution of a galaxy by star formation, stellar radiation and explosions, the modelling must include the dynamics of all these components and their interaction processes.

The pictures show the densities and velocities of the cool interstellar gas (temperatures between 100 and 3000 K) during the collapse of a dwarf galaxy after 0.5 billion years (upper row) and after two billion years (bottom row). The size of each figure amounts to 20,000 light years. The rotation axis points vertically along the left edge. The red central regions have the highest mass densities; blue is most dilute. The central density in the right-hand model (with dark matter) exceeds that of the left one by a factor of 10. The collapse proceeds faster in the right-hand model.

(Andreas Rieschick, Gerhard Hensler, Institute of Theoretical Physics and Astrophysics, University of Kiel)

Introduction	Super- computing	Astro- physics	Elementary Particles	Many Particles	Polymers	Chemistry	Environment	Other Fields