Stellar Convection - Magnetic Field - Pulsation

A proper treatment of turbulence still poses one of the most prominent problem areas in theoretical astrophysics. This phenomenon, observed in combustion engines as well as in supernova explosions, in our terrestrial atmosphere as well as in our sun, proved to be highly resistant to a satisfying modeling which is appropriate for the enormous scales in time and length in question. Additionally, the physics involved can be extremely complex. As it is rightly stated in Hillebrandt & Kupka (2009), "Although, in a sense, the dynamics of fluid flows is well understood since about 200 years, described by an innocently looking nonlinear partial differential equation, the Navier-Stokes equation, even giants of modern physics, such as Werner Heisenberg, are quoted saying that the two major unsolved problems of physics are the unification of quantum physics and gravity (on the one hand) and turbulence (on the other).". Strong links exist between plasma physics in astrophysics and human needs, mentioning here only space weather and the terrestrial magnetic field. The potential role of thermonuclear fusion as a resource of energy needed on earth would be another example or understanding how our sun works (with backlash on life on earth). Modern astrophysics has the advantage to allow investigating a temperature and density regime, which is barely - or even not at all - accessible in terrestrial laboratories, and hence to investigate important aspects of plasma physics, like convection and its interaction with magnetic fields under extreme conditions. All these arguments contribute to answer the question why it matters that we understand under which circumstances convection occurs in stars. Furthermore, deficits in our understanding of stellar structure and evolution limit our understanding of the chemical evolution and physical properties of the Milky Way and of other galaxies, with consequences for cosmology. An important scientific goal for the next decade is the detection and analysis of the "first stars" formed in the Universe after the Big Bang. Since it is likely that these first stars were massive and unstable, a better quantitative understanding of stellar models including convection, magnetic fields, opacity problems, etc., will be crucial. It is the goal of the present project to provide solid observational evidence for turbulent convection and magnetic field effects in plasma, and of their interaction. This evidence shall guide developing and testing numerical and/or analytical techniques used for modeling stellar interiors and evolution. It is clear that a complete convection theory will not be created in the near future, but the way definitely has to be paved in this direction .

We collected new observational evidence for convection, stellar magnetic field effects, rotation, and chemistry as a guide for developing and testing models of the interior and evolution of stars populating the central part of the Hertzsprung-Russell diagram (HRD). Particular focus was put on pulsating stars, because of their significance for investigating internal structures by applying asteroseismic techniques, similar to geo-seismology, where it is possible to derive the internal terrestrial structure from earthquakes.We investigated the borders of the respective instability strips as a function of fundamental stellar parameters, like Teff, log g, metallicity, pulsation amplitudes and frequencies and we put our results in context of current theories. The first most interesting result is a dependency of the cool border of the ??Sct instability strip from metallicity, which was already predicted by theory, but never tested with observations. Furthermore, we have clear evidence that this cool border deviates from model predictions as luminosity increases. With access to space photometry (MOST, CoRoT, and Kepler) we improved dramatically the statistically relevant sample size. The analysis of more than 6000 stars required the development of efficient tools for a reliable and quick data reduction beyond standard pip-line techniques and for deriving stellar fundamental parameters and pulsation frequency spectra. Presently, this work is documented in two PhD Theses that will be published in journals. We see also clearly hybrid pulsators deviating in their pulsation phenomenology from current models. Carefully selected individual key-targets from our large sample were investigated in more detail, including for some of them the determination of magnetic fields. In particular the interaction of a magnetic field with stellar atmospheres and structures was studied for individual stars. This part of our research is documented in more than 60 papers published in cooperation with external collaborators in refereed journals. Highlights are the discovery of the first convincing case of co-rotating bright spots on an O star (very hot and massive), the first discoveries of magnetic fields in two massive stars, the longest-period roAp star, the first very young ? Doradus type pulsator und two slowly-pulsating-B stars,An unplanned additional research aspect emerged from the obvious need of other consortia to determine fundamental parameters of planet hosting stars, if one wants to understand the generation and evolution of planets. This provides a very attractive additional opportunity for an application of all our newly developed tools. Obviously, the discovery of an exoplanet that is hosted by a bright star would allow application of highly specialized research methods, not feasible for a faint target. As we have direct access to the nanosatellite project BRITE-Constellation, we investigated also the possibilities for detecting exoplanets hosted by bright stars.