Evolution of planetary systems in binary stars

In the last years the rapidly increasing number of observationally detected planetary systems, their formation and dynamical evolution and especially their stability, became a special focus of astrophysical research. Among the recently discovered planets a small fraction is also found in binary stars like the well-observed prototypical system Gamma Cephei. Although most of the investigations concentrate on planets or protoplanetary disks around single stars, planets in binary stars are of special interest since more than 60% of the stars in the solar neighbourhood form double- or multiple star systems. We believe that understanding the influence of a secondary star on a planetary disk, and especially understanding which circumstances lead to the existence or absence of planets, will shed some light on the formation and long term evolution of these systems. The project`s aim is to extend state-of-the-art evolutionary modelling and stability analyses (both short-term and long-term) of planetary systems in double stars to cover also early stages of their evolution after their formation. The main goals are twofold, but strongly interconnected: First, the evolution of proto-planetary disks and already formed planetary systems in binary stars (i.e. when a circumprimary gas/dust disk is still present) will be studied by 2D and 3D fluid dynamical simulations. Second, long-term dynamical studies will be extended to include also the early evolutionary stages when the presence of a proto-planetary disk (or its remnants) is still important. The latter is based on a heuristic description of the interaction between disk and planet. This will be developed by a thorough comparison with the short-time fluid dynamical simulations. We expect to be able to make highly accurate long-term predictions of the dynamics of planetary systems including the interaction with a gas/dust disk. As a prototypical system for a binary star hosting a planet we will use Gamma Cephei as a reference model. The principle investigators have a large experience in orbital dynamics and stability analysis of planetary systems and in 2D and 3D fluid dynamical modelling with both, grid-based and smoothed-particle-hydrodynamics (SPH) codes. For flat self-gravitating multi-component disks a 2D-code has been developed and applied by one of the applicants. With the results obtained the applicants expect to derive - or at least to constrain - the properties of protoplanetary disks in some well-observed binaries. Since in the next years a strong increase in the number of detected planets in double star systems is expected - the European mission CoRoT, which was started end of December 2006, will soon provide us with new observational data about planetary systems, including binary systems, the results of the proposed project will represent a valuable and timely contribution to the research and understanding of extra-solar planetary systems.

This project analysed the formation and evolution of planets in binary star systems. While the formation scenario requires time-consuming hydrodynamical simulations the evolution after the gaseous phase can be examined via pure N-body computations. An important aim of this project was to combine these two fields to be able to perform hydrodynamic disc computations with highly precise N-body simulations -- which were not possible so far. The lack of such investigations might be related to the huge computational requirements. To overcome this problem we developed a GPU-CPU 2D hydrodynamic grid code which is able to handle a huge number of proto-planets (more than 2000) and a gas-disc simultaneously. The tremendous computational power of graphic cards allowed us to run such simulations which are not feasible just using a normal CPU. Before performing formation scenarios for different binary star systems we had to examine various boundary conditions which are important especially for such studies in binary star systems. The dynamical part of this project as well showed interesting new results. We have shown that for circum-stellar motion of a planet the eccentricity and the mass of the planet are important for the extension of the stable area. This means the higher the eccentricity and the mass the stronger is the reduction of the stable area compared to that of a simplified model called restricted problem where one studies quasi mass-less planets in the gravitational field of the two stars. Our study is important since all general stability studies that are commonly used were performed using this simplified model.Another important result of this project was the study of terrestrial planets in the habitable zone in circum-stellar motion. The computations of such a planet in a binary star system using different eccentricities for the stars indicated a periodic variation of the insolation on the terrestrial planet. When analysing this result in detail we have found the same variation in the eccentricity of the terrestrial planet. This means the gravitational perturbations of the secondary star forces the planet to orbit its host-star in a more or less eccentric orbit -- depending on the masses of the stars, on their distance and their eccentricity. It is obvious that an elliptic orbit of a terrestrial planet leads to more or less strong variations in the insolation depending on the parameters of the system. These variations are certainly important for studies of habitability -- that are very popular at the moment -- as strong variations in the insolation could avoid the habitability of a planet.Since we know that binary star systems are more frequent in our Galaxy than single stars the studies of this project provide an important contribution to actual research topics in astrophysics.