Turbulent Convection and Pulsation Interaction in Stars

In many stars energy is transported from their interiors outwards by both radiation and convection. For the Sun the region where convection occurs extends from its surface down to a domain located at 30% of the distance from its uppermost layers to its centre. This outer region of a star is called stellar envelope. Convective envelopes are typical for cool stars which produce their energy directly in their centre by nuclear fusion of hydrogen or in a region located just immediately outside of it, as in the so-called red giant stars. Convection not only causes energy transport and mixing in these objects. It also excites these stars to oscillate. This has been demonstrated particularly with measurements which have been made from special satellites such as COROT, Kepler, MOST and also with the help of the BRITE satellites. Convection gives rise to processes which may excite oscillations, but it can also damp them. This competition of different processes can be analysed by continuous observations of stars preferably made over many days without any interruptions. Therefore, the pulsations, which appear as small temporal variations of stellar brightness, are measured with high precision. The strength and frequencies of the oscillations determined this way and averaged over longer periods of time allow conclusions on the basic properties of a star. Age, information about the original and current chemical composition, as well as the mass, the luminosity, and the radius of a star can be determined this way. This is of particular interest for the study of planets around other stars and the study of our Galaxy, since they allow drawing conclusions on their environment and evolution in time. To achieve the necessary accuracy for this approach, an accurate qualitative and quantitative understanding of convection and its interaction with the pulsations it drives is necessary. This is the research topic of this project for which high resolution computer simulations over sufficiently long time intervals, hundreds of oscillation periods, have to be performed. This corresponds to a total time ranging from a day up to two weeks. Thereby the flow processes for a small region at the stellar surface are investigated by approximate, numerical solution of the hydrodynamical equations and compared to predictions from simpler models derived from turbulence theory. Also the vertical oscillations that occur within such simulations will be compared to observations of their counterparts in stars. This will require both improvements of mathematical methods as well as of the simulation models. The results from this work will be an important part for the preparation of the PLATO 2.0 satellite mission which aims at an exact characterization of a very large number of stellar planetary systems.

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