Fischer, Christian 
ORCID: 0000-0002-7299-8661
(2020).
Time-Variable Electromagnetic Star-Planet Interaction in the TRAPPIST-1 System.
PhD thesis, Universität zu Köln.
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Abstract
Electromagnetic Star-Planet Interaction is the process, when planets in orbit around a star, couple to the star via the stellar magnetic field. The relative motion of the planet through the stellar wind plasma generates magnetohydrodynamic waves. If the stellar wind velocity at the planet is smaller than the local Alfvén speed, the generated Alfvén waves can travel upstream, against the plasma flow, towards the star. These waves establish a coupling between planet and star and transfer energy towards the star. In our solar system, we have no star-planet interaction, because all planets are too far away from the sun to generate such a coupling. Instead, all planets generate bow shocks. However, the large moons of the giant planets in our solar system generate the similar effect of moon-magnetosphere interaction. A problem of star-planet interaction is that it is hard to observe. The bright background of the stellar emissions further complicates a definite identification. Several observational studies found enhanced emissions in certain spectral lines of stars. However, it is unknown, what type of emissions star-planet interaction generates in stellar atmospheres. Therefore, one needs a further indicator that a planet generates the observed emissions. Temporal variability of the star-planet interaction can provide this missing information. Previous studies have looked for signals that appear with the planetary orbital period. We show that temporal variability has a much larger variety than just the orbital period, which we assume as the simplest mechanism for variability. Three additional mechanisms can account for periodic variabilities with their distinct period. Tilted stellar dipole fields generate signals with half the synodic rotation period of the star as seen from the planet. Magnetic anomalies on the star may be triggered by star-planet interaction to erupt flares periodically with the synodic rotation period. The fourth proposed mechanism assumes an interaction between the star-planet interaction of two planets that would appear with the synodic rotation period between both planets. We call this process wing-wing interaction. For our studies, we choose the TRAPPIST-1 system, because its seven close-in planets make the system a perfect candidate for the search of star-planet interaction. In the following, we conduct a semi-analytic parameter study to determine which planets could generate star-planet interaction. According to this study, the two innermost planets are the best candidates for the search of star-planet interaction. To understand the interaction better, we conduct time-dependent magnetohydrodynamic simulations of star-planet interaction. Our results show that the wave structure going towards the star is indeed purely Alfvénic and the power resembles the analytically predicted value very well. The waves that go away, however, comprise Alfvén waves, Slow Mode waves, Entropy waves and a Slow Shock. Those waves may affect the interaction of outer planets. We investigate the scenario of wing-wing interaction with both inner planets of the TRAPPIST-1 system. The waves of the inner planet dissipate the coupling wave structure of the outer planet. Later, the compressional wave modes affect the interaction of the outer planet. Due to this simulation, we had to improve our proposed model for wing-wing interaction. Our model setup also allows inhomogeneous stellar winds with coronal mass ejections. The mutual interaction between star-planet interaction and coronal mass ejection has not been investigated before. Our simulations show that the coronal mass ejection dislocates the coupling Alfvén wave structure. In the final step of this thesis, we analyse the flares of TRAPPIST-1 that we have read out from a published light curve observed by the K2 mission. We assign each flare a duration and calculate Fourier transform and autocorrelation of the time series. Additionally, we test the significance of the results with statistical tests. These tests show that the obtained result indeed points at flare triggering by interaction with TRAPPIST-1 c.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-252008 | ||||||||
| Date: | 2020 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology | ||||||||
| Subjects: | Physics Earth sciences |
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| Date of oral exam: | 11 September 2020 | ||||||||
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| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/25200 |
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