Numerical Simulation of A Stars and White Dwarf Stars
Astrophysical research plays an essential role in the advancement of modern physics. It influences our understanding of the fundamentals of physics as well as the possibilities of applying physics to scientific and technical questions.
This is because all sub-disciplines of physics are important to astrophysics. The innovations motivated by it include, for example, the development of new measurement techniques for ground-based telescopes and instruments deployed in space, and the creation of computer-based models of complex physical processes to understand the Sun. It also leads to questions concerning the foundations of physics itself, such as whether our idea of how gravity works is correct. In the same way, astrophysics motivates the further development of state-of-the-art mathematical methods such as those used in computer simulations.
For example, a numerical solution method developed for the investigation of mixing processes in the interior of massive stars, as they also occur in the interior of planets or in oceans and lakes on Earth, can be used not only for research in astrophysics and geophysics, but also in biology, chemistry or financial mathematics. This is because processes that initially appear to be something completely different can be investigated using the same solution methods due to a similar mathematical structure, which can thus be transferred to practical applications. This is the case, for example, in the risk assessment of investment portfolios by banks and insurance companies or in the construction of solar ponds for the generation of electrical energy.
The “Numerical Simulation of A Stars and White Dwarf Stars” project, funded by the Austrian Science Fund (FWF), focuses on the study of convection in the aforementioned object types. Their selection is motivated by a number of unresolved issues within the discipline of stellar astrophysics, but also by the fact that the generation of numerical simulations for these objects is also particularly well suited for the further development of numerical methods. Convection is an important physical process for transporting heat in liquids, in gases and plasma. It can mix them rapidly and cause a variety of hydrodynamic phenomena in them or alter their flow. These include the formation of large upflow and downflow structures (granules and plumes), running or standing waves and shock waves. The challenge in computing these processes in stars is that they are usually very turbulent. A more detailed understanding of these processes requires the numerical solution of the physical conservation equations and, based on this, the performance of numerical simulations of convection in stars. Even with powerful supercomputers, not all processes can be taken into account: The simulations are constructed to spatially and temporally capture the processes most important for physical understanding. To achieve this, the further development and application of new numerical methods is also necessary.
In this project numerical simulations of sections of the surface of stars will be performed. Stars of spectral type A and white dwarf stars of spectral type DA will be the focus. After all, why only some of the cool A and metal-rich Am stars pulsate is as yet poorly understood, as is the precise role that convection has in exciting and damping global oscillations in these stars, even though high-precision data are available for many of these objects through space missions such as Kepler and TESS. Similarly, convection mixes apparently stably layered regions in such stars, and this process is particularly effective in the types of stars being studied. The role that turbulence plays in this so-called overshooting (mixing beyond the stability limit) is still insufficiently unexplored. Therefore, a whole series of numerical simulations of A stars will be performed and in this way a first model grid for these objects will be established. The key to new results will be a high spatial resolution through local grid refinement, as made possible by the simulation code ANTARES. For A stars, this requires the implementation of more advanced numerical methods (implicit-explicit Runge-Kutta methods) so that the temporal resolution of the simulation does not become unfeasibly small. These tools will then be used to investigate the role of turbulence in overshooting in white dwarf and A stars, how the turbulent pressure created by convection affects the occurrence of global oscillations in A/Am stars, how turbulent convection alters the spectra of these stars, and how spatially separated convection zones connect in cool A and hot F stars. The predictions of the simulations are tested with various observational methods.
