Abstract
Since the first detection of a circumbinary planet with the Kepler space telescope in 2011 nine more circumbinary planets have been discovered. All these circumbinary Kepler systems have two things in common: First they are very flat, meaning that the orbit of the binary and the planet are in one plane, suggesting that they formed in an accretion disc surrounding both binary components. Second, the orbits of the planets are very close to the calculated stability limit. To explain the formation of these planets two scenarios are possible: An in situ formation at the observed location or a formation further outside in the disc followed by radial migration to the current observed position. Simulations have shown that an in situ formation is unlikely, due to destructive collisions of planetesimals on orbits close to the binary. The second scenario leads to the question of how the migrating planets can be stopped at the observed location. In this thesis the second scenario is examined through numerical simulations. The gravitational interaction between the binary and the disc leads to the formation of an eccentric inner gap, which precesses slowly in a prograde manner around the binary. This inner gap constitutes a barrier for a migrating planet. Therefore, the first part of this thesis investigates how binary parameters (eccentricity, mass ratio) as well as disc parameters (pressure, viscosity) influence the size, eccentricity, and precession period of the gap. The binary eccentricity (ebin) is identified as an important parameter. In the precession period – gap-size diagram a bifurcation occurs for varying ebin. Increasing the binary eccentricity from zero, precession period and gap-size decrease until a critical eccentricity of ebin = 0.18 is reached. From this point onward, precession period and gap-size increase again for increasing binary eccentricities. The binary mass ratio changes only the precession period, which decreases with increasing mass ratios, while the gap-size remains constant. For increasing viscosity and pressure in the disc the expected behaviour is observed: precession period and gap-size decrease. The second part of this thesis investigates the migration of planets in circumbinary discs. For five Kepler systems (Kepler-16, -34, -35, -38, -413) the dependence of the final orbital parameters on the planet-to-disc mass ratio is examined as well as the change in the disc structure due to the presence of the planet. The planets migrate in all cases to the edge of the gap, as expected. Depending on the planet mass, two migration scenarios are observed. Massive planets dominate the disc by shrinking and circularising the inner gap, whereas light planets are influenced by the disc. They align their orbits to the precessing disc and their eccentricity is excited. In general the final simulated orbital parameters are too large compared to the observations. Circumbinary planets around systems which create large, eccentric gaps (Kepler-34 and -413) also have the highest simulated eccentricities in agreement with the observations.
Author and committee
dc:creator, dc:contributor.*- Author
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- Thun, Daniel Gerd
Identifiers
dc:identifier.*- Identifier
- hdl:10900/86529