The origin and evolution of warm exozodiacal dust
Many stars show excess mid-infrared emission which is attributed to warm dust in the habitable zone of the star, known as exozodiacal dust, or exozodis for short. Such dust will be a source of noise and confusion when attempting to detect and characterise Earth-like planets. Therefore, an understanding of exozodiacal dust is crucial to our search for habitable planets and life. In this thesis, I present theoretical models for the origin and evolution of warm exozodiacal dust. Observations find a strong correlation between the presence of warm habitable zone dust and cold belts of planetesimals similar to the Solar System’s Kuiper belt. Given this correlation and the short lifetime of dust grains close to the star, it is probable that exozodiacal dust originates further out in the planetary system and is transported inwards. One possible transport mechanism is Poynting-Robertson (P-R) drag, which causes dust grains to lose angular momentum and spiral in towards the star. Initially, I develop an analytical model for the interplay of P-R drag and catastrophic collisions in a debris disc which predicts the levels of exozodiacal dust dragged into the habitable zone of a star from a cold outer belt. I show that detectable outer belts should produce exozodi levels tens of times higher than our zodiacal cloud via P-R drag, but these levels are insufficient to explain a large fraction of exozodiacal dust detections. In-depth application of the model to the exozodi of β Leo suggests the presence of an additional, warm asteroid belt to explain the radial profile of habitable zone dust. An alternative mechanism is inward scattering of comets, which spontaneously fragment to produce dust. I then develop a numerical model for the zodiacal dust produced by spontaneous fragmentation of Jupiter-family comets in the Solar System. This is able to produce enough dust to sustain the zodiacal cloud, and give the correct radial and size distribution of dust. I show that cometary input to the zodiacal cloud should be highly stochastic, depending on the sizes and dynamical lifetimes of comets scattered in. The comet fragmentation model is then extended to be applicable to other planetary systems, taking into account the different dynamical effects. This model will show how much dust comets produce and its evolution after being released from a comet to give exozodi radial profiles. Finally, I summarise the work in this thesis, and discuss the future outlook and my planned projects for furthering our understanding of exozodiacal dust.
Astronomy, Exoplanets, Solar System, Small bodies, Exozodiacal dust, Zodiacal dust, Comets
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