Numerical Modelling in Astrophysics

Numerical modelling takes a more and more important place in astrophysical research. Although normally not the astrophysicists develop new mathematical algorithms, they often initiate new research in the field by combining various methods.

Calculation of the formation of a jet like structure due to a stellar outflow and collimation by a torus of material, as it is formed around rotating stars. This disk (light blue) has been nearly destroyed. Nevertheless the ejected material keeps fairly collimated (red). ( Credit: Felix Niederwanger)

Numerical simulation means for us – splitting the continuous (infinite) structure in nature in a finite number of discrete cells, assuming constant physical properties within this sub-region. These methods are applied also in Metrology, Oceanography, etc.

Astrophysicists have extreme ranges for their physical properties. Talking about hydrodynamics as an example: The densities vary in these investigations from regions which would be called in laboratory physics ultra-high vacuum, up to densities and pressures which never can be produced in labs. The geometrical scales require so called sub-grid simulations. Often the source of energy for a gas cloud is that small, that it would not even fill a single cell in the simulations (e.g. a white dwarf star in a planetary nebula or in a Nova system or galaxies in simulations of the X-ray emitting intracluster medium).  It can’t be resolved in the simulation even with the biggest supercomputers available. Thus special investigations are required, to fit this into the standard mathematical schemes.

The carbonic acid di-mer recently shown to exist in gas phase (credit Huber et al. doi: 10.1063/1.4755786). Numbers are bond lengths in Ångstrom and bond angles in degrees.

Other investigations deal with molecules in space. Even simple molecules like carbonic-acid were assumed not to exist as such in gas phase. In joint effort with groups doing theoretical quantum-chemical calculations it was shown, that it may exist as an electrically coupled di-mer in environments like the high atmosphere of the earth or above the polar caps of Mars. Same applies for distorted molecules. Low densities and high velocities of impacting hydrogen or helium atoms may cause deformation of organic molecules. The new, more precise calculations of the electronic states in these molecules allow more accurate predictions, at which infrared wavelengths spectral lines should exists. It is now for the observers to investigate them in their spectra. 

Faculty member active in this area: Stefan Kimeswenger