Putting the "Planets" back into "Planetary Nebulae"

This thesis investigates the hypothesis that planetary engulfment during the asymptotic giant branch (AGB) phase of planetary nebula progenitor stars contributes significantly to the formation of non-spherical planetary nebulae (PNe), building on the binary-shaping hypothesis. Planetary nebulae form as stars expel their outer layers, leaving behind an expanding shell of gas. The majority of PNe exhibit asymmetrical, non-spherical shapes, suggesting external influences in their formation. The research investigating the origin of their shapes was conducted using a tool called SIMSPLASH (SIMulationS for the PLAnet Shaping Hypothesis), which is a research code that calculates the orbital evolution of exoplanets by combining the host stars’ stellar mass-loss rates with the tidal evolution of the bodies involved. Via this mechanism, the probabilities of stars having and engulfing massive planets as a function of stellar mass could be determined. The study employs data from currently discovered exoplanets, updating any previous statistics, as well as incorporating three-body systems of different varieties into the analysis. Updates were also made to the code that allowed for a broader and more efficient exploration of the hypothesis. The total rate of engulfments on the AGB is 6.5%, and is just slightly higher than that for only single-star single-planet systems with a rate of 5.6%. Neither of these percentages provides strong support to the idea that planet engulfment accounts for the vast majority of aspherical PNe shaping. However, some of the observation and selection biases, such as a very high engulfment rate in higher-mass stars but a lack of discovered progenitor systems with host stars of such masses, provide additional context for the hypothesis. Additionally, the results show that the majority of planets are engulfed on the RGB, of which the effects on the stellar evolution thereafter have not yet been fully researched. The overall conclusion that can be drawn from this work is that while planetary engulfment can explain a small portion of non-spherical PNe, the evidence currently suggests that it is not the primary mechanism behind the shaping of the majority of these nebulae. Therefore, the hypothesis is not sufficient enough to resolve the mystery of non-spherical planetary nebulae morphologies.

Publisher

University of Galway

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Attribution-NonCommercial-NoDerivatives 4.0 International

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University of Galway Theses (Research Masters Theses)