Doctor of Philosophy in Interdisciplinary Studies (PhD)
The sustainability of the outer space environment
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Empirical trends in stellar X-ray and radio luminosities suggest that very low massstars and brown dwarfs should not produce significant radio emission. Defyingthese expectations, strong non-thermal emission has been observed in some UCDsin the 1−10 GHz range, often attributed to global aurorae. At higher radio frequen-cies, flux due to global aurorae becomes unphysical, and is instead attributed togyrosynchrotron radiation. In my Ph.D. work I used observations in this frequencyrange (30−100 GHz) to infer the presence of gyrosynchrotron radiation from ultra-cool dwarfs in three stars, and to place upper limits on the radio flux on three otherstars that were not detected. Prior to this work, only one ultracool dwarf had beendetected at such high radio frequencies. My results suggest that gyrosynchrotronradiation from radio-loud ultracool dwarfs might be more common than previouslyassumed. Another key component of my thesis has been an extensive radio studyof the ultracool dwarf TRAPPIST-1. The TRAPPIST-1 system is notable for itsseven terrestrial planets, at least three of which orbit within the habitable zone. Iput upper limits on the quiescent radio emission from the star at 44 and 97.5 GHzusing the VLA and ALMA, and monitored the long-term 2 GHz emission fromthe star over 50 hours to search for variability. I used these results to constrainthe possible gyrosynchrotron radiation from the star, as well as the resulting spaceweather impacts on surrounding planets. My results from the TRAPPIST-1 studiessuggest that the TRAPPIST-1 planets are not regularly exposed to high popula-tions of energetic particles due to stellar activity, supporting the case for planetaryhabitability.
The discovery of exoplanets on short orbital periods (P 10 Earth-masses. We compare the dynamical outcomes of gas-free and gas-embedded planetary systems, in which consolidation of a critical core was only possible in the gas-free simulations. In contrast, STIPs are resistant to instability when gas is present, resulting in coplanar and nearly circular systems. The instability of the configurations after 10 Myr increases if the eccentricity is perturbed to e~0.01. In some cases, the planet-disk interaction produces co-orbiting planets that are stable even when the gas is removed. We explore the transit detectability of these configurations and find that the coorbital transit signature is difficult to identify in current transit detection pipelines due to the system dynamics. To explore STIP evolution in the presence of an outer giant planet, we vary the semi-major axis of the perturber between 1 and 5.2 au. We find that the presence of the outer perturber, in most locations, only alters the STIP precession frequencies but not its evolution or stability. In those locations where the perturber causes secular eccentricity resonances, the STIP becomes unstable. Secular inclination resonances can affect the observed multiplicity of transiting planets by driving the orbits of one or more planets to inclinations about 16 degrees.
Debris discs are systems of dynamically evolved byproducts of the planet formation process. They can be used to test various planet formation theories. In my thesis I use submm-cm observations to characterize the debris in HD 141569 and Fomalhaut, as well as to investigate how stellar emission can serve as a confounding parameter in disc studies. HD 141569 is a unique system hosting a large B9.5 star, a complex circumstellar disc of gas and dust, and two M dwarf companions. Using ALMA data, I inferred the total gas mass of the system and directly imaged the inner and outer edge of the gas disc. Using ALMA and VLA data, I placed constraints on the morphology, mass, and dynamical state of the inner and outer dust discs. I used the properties of the gas and dust to argue that the system may be more accurately characterized as a young debris disc as opposed to a transitional disc. Fomalhaut is a commonly studied nearby debris system. I used ALMA observations to place tight constraints on the morphology, mass, and grain size distribution of the outer debris ring. In addition, I used ALMA and IR data to cast doubt on the existence of an asteroid belt in the inner system. To separate the emission from discs and their host stars, high angular resolution observations are necessary. When the resolution is still not sufficient to spatially separate the two, an accurate model of the stellar emission is required. I am the PI on an observational campaign entitled Measuring the Emission from Stellar Atmospheres at Submillimeter/millimeter wavelengths (MESAS). This project seeks to observe stars with no known debris at wavelengths commonly used for studying discs, build a spectral profile of the sub-millimetre to centimetre emission, and use these profiles as templates for the stellar emission in unresolved debris features.
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
The growth and migration of planetesimals in a young protoplanetary disk is fundamental to the planet formation process. However, in our modeling of early growth, there are a several processes that can inhibit smaller grains from growing to larger sizes, making growth beyond size scales of centimeters difficult. The observational data which are available ( e.g., relics from asteroids in our own solar system as well as gas lifetimes in other systems) suggest that early growth must be rapid. If a small number of 100-km-sized planetesimals do manage to form by some method such as streaming instability, then gas drag effects would enable such a body to efficiently accrete smaller solids from beyond its Hill sphere. This enhanced accretion cross-section, paired with densegas and large populations of small solids enables a planet to grow at much faster rates. As the planetesimals accrete pebbles, they experience an additional angular momentum exchange, which could cause slow inward drift and a consequent back-reaction on growth rates. We present self-consistent hydrodynamic simulations with direct particle integration and gas-drag coupling to estimate the rate of planetesimal growth due to pebble accretion. We explore a range of particle sizes and disk conditions using a wind tunnel simulation. We also perform numerical analyses of planetesimal growth and drift rates for a range of distances from the star. The results of our models indicate that rapid growth of planeteismals under our assumed model must be at orbital distances inwards of 1 AU, and that at such distances centimeter-sized pebblesand larger are required for maximized accretion. We find that growth beyond 1 AU is possible under certain limited, optimized conditions.