The Effects of Stellar Magnetic Activity and Variability on Observations of Exoplanets
Morris, Brett Michael
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The search for life in the Universe outside of the Solar System focuses on the study of potentially habitable exoplanets. Exoplanets have been discovered principally via the transit method, which reveals planetary radii and orbital periods, among other things. In transiting multi-planet systems, we can measure the masses of exoplanets without expensive spectroscopy by using transit timing variations. The orbit of a single transiting planet around a single star would be (nearly) perfectly periodic, but if there's more than one planet in the system, the gravitational influence of each planet on each other changes the orbital shape and orientation. The apparent variation in the exoplanet transit times thus transmits information about the mass and orbit of the perturbing planet. Transits can also reveal the composition of exoplanet atmospheres via transmission spectroscopy. When the planet passes in front of the host star, it will appear largest at wavelengths where the planet's atmosphere is opaque, and smallest at wavelengths where the atmosphere is transparent. Thus by measuring the apparent radius of the planet as a function of wavelength, we can obtain a spectrum of an exoplanet's atmosphere. However, there are additional signals generated by magnetic activity and variability at the stellar surface which inject confounding time- and wavelength-dependent signals into the spectrophotometry of exoplanet host stars which complicate all of the aforementioned measurements. We must understand heterogeneous stellar surfaces in order to accurately answer astrobiological questions such as: does this planet have a surface, and what is the composition of its atmosphere? The goal of this dissertation is to explore stellar magnetic activity and variability and their impacts on measurements of exoplanets with implications for astrobiology. I used the transiting planet HAT-P-11 b to measure the size and latitude distributions of starspots on its active K4 dwarf host star, to find that its magnetic activity mirrors the Sun's. I measured the chromospheric activity of HAT-P-11 and compared it to stars of similar temperature and rotation period to find that it's the most active of its peers, perhaps suggestive of star-planet interaction. I directly measured starspot coverage on a sample of bright stars via TiO molecular band modeling. I identified the possibility of detecting stellar activity cycles of nearby stars using precision astrometry. I devised a technique for measuring robust exoplanet radii even in the presence of significant starspot distributions by reparameterizing the transit light curve of Mandel & Agol (2002). Finally, I devised a simulator for James Webb Space Telescope observations of transiting exoplanets, to explore the limits imposed by stellar magnetic activity on transit timing and transmission spectroscopy measurements for three systems with potentially habitable exoplanets.
- Astronomy