Stars Eating Planets

Detta är en Master-uppsats från Lunds universitet/Astronomi

Sammanfattning: In a phase of instability during the dynamical evolution of some multiple exoplanetary systems, exchange of angular momentum and/or energy through gravitational interactions between planets will lead to alterations of their orbital properties. In some cases, planetary orbits end up crossing, which leads to close encounters between two planets. In turn, they undergo a planet--planet scattering event, where a large exchange of angular momentum and energy leads to a significant change in trajectories and thereby their orbits. The distribution of eccentricity for observed giant planets provides strong evidence for past planet--planet scattering events in many exoplanetary systems. One particular outcome of such an event is when one of the planets ends up on a highly eccentric orbit, which leads to a planet--star collision, also referred to as planet consumption. When a planet is consumed, it can transfer physical quantities such as angular momentum, energy and heavy elements to the host star, altering its properties. Due to the transfer of said quantities, a planet consumption event can have observational consequences for the host star that are detectable by astronomical instruments. In this thesis, I first employ a semi-analytical two-body model to constrain which type of orbital configuration in a planetary system with a single star will facilitate planet consumption. I then use the constrained parameter space to formulate a fiducial planetary configuration. The dynamical evolution of said system is then modelled using 100 numerical N-body integrations, which allows me to further determine for which type of systems that planet consumption by planet--planet scattering is possible. Moreover, I tie the results of the semi-analytical and numerical analysis to a literature study in order to constrain which type of observational consequence will dominate for consumption of planets on highly eccentric orbits. From the two-body analysis, I conclude that planet consumption is more probable for scattering events where: the planetary mass ratio is extreme, where the inner planet is less massive than the outer; the planets are orbiting a host star with low density; at least one of the orbits is highly eccentric, preferably that of the least massive planet; the event occurs at small separations from the host star. Based on these results, I also formulate the fiducial planetary system, which is a Solar System analogue with two Earth-mass planets inside of 1 AU and three initially unstable Jupiter-mass planets beyond 5 AU. From the numerical N-body integrations I find that hierarchical systems with low-mass planets and at least two unstable giant planets will consistently induce consumption of planets of 30 Earth masses and less. When the system has three giant planets, around 10% of the integrations lead to the consumption of a giant planet. Such an event can produce an observational consequence where a single giant planet ends up on a very distant orbit with arbitrary eccentricity. The integration results also show that there are three extreme pathways to planet consumption: diffusive planet consumption, where the eccentricity of a planet increases diffusively over a large number of scattering events; strong planet consumption, where the eccentricity is boosted up quickly over a small number of scatterings and Lidov--Kozai planet consumption, where the eccentricity increases through the Lidov--Kozai mechanism which excludes planet--planet scattering. In the literature study I determine that the dominant observational consequence highly depends on stellar properties such as age, metallicity, mass and radius, as well as planetary mass, radius and composition. Moreover, the minimum separation between the planet and the host star during an orbit determines the strength of detectable signatures. A majority of the observational consequences are difficult to directly tie to planet consumptions, meaning that detections of such events are good targets for future multi-waveband astronomy missions. From the results of the numerical integrations performed, I estimate that the dominant observational consequence from planet consumption in the Milky Way is metallicity enhancement by consumption of super-Earths. Outside the Galaxy, the dominant observational consequence is planet merger transients caused by the consumption of a giant planet, which induces an increase of stellar luminosity in the optical/infrared wavebands followed by a radio afterglow that lasts for a few thousand years.

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