Dissertation

Applications of Transit Surveys to Red Dwarfs

Foxell, Emma

2020

Overview

Many hot Jupiters have been discovered from the ground and Kepler discovered many exoplanets around fainter stars, but searches push for more smaller planets around bright stars. Transit photometry also relies on knowledge of host star properties, less certain for later spectral types. This thesis aimed to investigate detecting planets smaller than Jupiter around bright, later spectral types and to help better understand M dwarf properties. NGTS aims to discover Neptune-sized exoplanets around K and M dwarfs. In Chapter 3, I investigated the brightest stars, saturated in the standard pipeline. While relatively few, these targets are highly valuable. I investigated whether saturated star fluxes could be recovered by using custom apertures, finding tailored rectangular apertures captured them well. I show the difference in noise between bright unsaturated stars and saturated stars is reduced using rectangular apertures rather than the standard pipeline, and is the same for a number of cases. However, to perform saturated star photometry, NGTS needs to change its operational gain setting, to allow charge to be conserved. Earth-sized habitable zone exoplanets are easier to detect around later spectral types as smaller, cooler stars are more favourable for transit photometry. However, their exoplanets are difficult to find in wide-field transit surveys due to their inherent comparative faintness. For this reason, I conducted a targeted survey of mid-late M dwarfs, presented in Chapter 4. Six relatively bright M dwarfs with spectral types M5−M8 were intensely observed, aiming for 50% phase coverage into their habitable zones. Some late M dwarfs also exhibit high flaring frequencies, so for the highest flaring rate target, I investigated the habitability for any habitable zone planets, finding them likely uninhabitable. Stellar properties are a large source of potential uncertainty in determining exoplanet properties, especially for mid-to-late M dwarfs, where stellar evolutionary models show inconsistency with observed data. Testing these models requires independently determined parameters, which is possible using EBLMs from transit surveys. In Chapter 5, I fit two EBLMs from SuperWASP. I find both secondary stars sit close to the bottom of the main sequence, where there are few other measured stars. However, properties of the higher mass (primary) star can still have large uncertainties, affecting uncertainties for the secondary star, and values vary depending on how primary star mass and radius are determined. Overall, this thesis shows some ongoing areas of interest within transit surveys. I determined how saturated star fluxes in NGTS could be used to search for valuable transiting exoplanets around bright stars. I conducted a survey of relatively bright mid-to-late M dwarfs specifically searching for Earth-sized exoplanets within their habitable zone. While no exoplanets were found, transit injections showed that we were sensitive to these planet sizes around at least some of our targets. I also fitted two low mass eclipsing binaries with the aim of better informing low mass stellar evolutionary models. I find that both secondary stars have potential but increased precision on their primary star properties is still needed.

• Publisher: ProQuest Dissertations & Theses