Astrophysics

Research at Jacobs University Bremen:

ESS at Jacobs is involved in a number of research projects with research institutes such as Cambridge University, various Max-Planck Institutes and the Institute for Advanced Studies Princeton).

We also enjoy a fruitful interaction with Geophysics, Computer and Computational Science, Applied Mathematics and the Oceanographic Sciences here at Jacobs University. Current staff members are interested in the following fields of research:

* Computational Astrophysics
* High-energy Astrophysics (neutron stars and Active Galactic Nuclei)
* Computational Fluid Dynamics
* Stellar Explosions (supernovae and gamma-ray bursts)
* Radio Astronomy
* Intergalactic Medium and Clusters of Galaxies
* Solar Physics and Solar-terrestrial Interactions

Research: Hydrodynamical Simulations

Most fields of astrophysics, such as solar physics, star formation, stellar collisions and explosions and cosmology have benefited greatly from hydrodynamical simulations over recent years and hopes for further advances are high.

There are two main approaches to the numerical solution of the equations of hydrodynamics that are commonly used in astrophysics: grid-based ("Eulerian") and particle-based ("Lagrangian") methods (in astrophysics a very popular method is the so-called Smoothed Particle Hydrodynamics method, SPH). In the former approach the equations are discretised on a computational mesh and flows between grid-cells are calculated. The latter method avoids the notion of a mesh and instead moves discretised portions of the flow ("particles"). Both methods have different strengths and weaknesses, the "right choice of method" usually depends very much on the problem to be solved. Generally, sharp continuities are easier to resolve with a grid-based method while the particle methods have advantages in numerically conserving quantities that are physically conserved (such as energy or angular momentum). In our group we employ both types of methodologies.

In recent times our group has performed several simulations that include magnetic fields. Codes that are regularly used are the adaptive mesh-refinement code FLASH (from the Center for Astrophysical Thermonuclear Flashes at Chicago), the grid-based MHD-codes PENCIL (Brandenburg and Dobler)and BATS-R-US (developed at the University of Michigan), the smoothed particle hydrodynamics code GADGET (developed by Volker Springel at MPA Munich) and the smoothed particle magnetohydrodynamics code MAGMA developed for compact objects (Rosswog and Price).

Research: Supernovae

Furthermore, we are involved in projects with the hydrodynamics group at the Max-Planck-Institut für Astrophysik which is concerned with type Ia supernovae. Because calibrated light curves of Type Ia supernovae have become a major tool to determine the local expansion rate as well as the geometry of the Universe, there is a considerable interest in modelling these events. It is believed that most type Ia supernovae are the explosions of white dwarfs that have approached the Chandrasekhar mass and that are disrupted by thermonuclear fusion of carbon and oxygen. However, the mechanism whereby such white dwarfs explode remains uncertain. No consensus has been reached on whether the star explodes as a result of a subsonic nuclear deflagration that becomes strongly turbulent or whether this turbulent flame phase is followed by a delayed detonation.

A fraction of type Ia explosions may also have a different origin: the coalescence of two white dwarfs in a binary system. Whether such a collision really produces a type Ia supernova is at the moment not clear, but as White Dwarfs are the "corpes" of the most common type of stars (low-mass stars like our sun) such collisions will definitely happen at a decent rate. Such a collision is currently investigated in a PhD-project (M. Dan).

Research: coalescences of neutron stars and black holes in binary systems

Neutron stars and stellar-mass black holes are the remains of massive stars that die in a supernova explosion. Neutron stars typically have masses around 1.4 solar masses, but a radii of only about 10 km. Therefore, they have huge matter densities, in their centres the density exceeds the density inside an atomic nucleus (2 10^{14} g/ccm) by factors of a few.

In our own galaxy, the Milky Way, to date eight binary systems consisting of two neutron stars are known. While they spiral around their common centre of mass the emit gravitational waves and therefore slowly spiral towards each other. This decay is in accurate agreement with the predictions from Einstein's Theory of General Relativity (Nobelprize for Physics 1993: R. Hulse and J. Taylor). Such gravitational waves are hoped to be detected directly (rather than indirectly as inferred from the orbital dynamics of a binary system) in the near future by ground-based Gravitational wave detectors such as the American LIGO project or the German British GEO600. Once the two neutron stars collide they probably cause on of the brightest explosions in the universe: a (short-type) Gamma-Ray Burst.