Discovery and Characterisation of M-dwarf Binaries and a Transiting Brown Dwarf with the Next Generation Transit Survey

One of the key goals of large scale exoplanet surveys is the search for “Habitable” Earth Sized planets. To this end, we are beginning to search for planets around low mass stars (M-dwarfs), as their small radii and cool temperatures are amenable to the discovery of rocky temperate planets. This presents a problem however in that unlike their sun-like counterparts, lower mass stars remain relatively poorly understood - particularly in terms of their mass-radius relationship. When measured masses and radii of M-dwarfs are compared with theoretical models, we observe discrepancies of up to 10% in some cases. As it is these parameters that are used to infer the structure of discovered planetary companions, this is clearly an issue. To help resolve these discrepancies, we can use precise measurements of stellar masses and radii from eclipsing binaries to refine the models. However the number of measurements in this region of parameter space is relatively small.

In this thesis I present the discovery and characterisation of low mass eclipsing systems from the Next Generation Transit Survey (NGTS). In the first two science chapters I present two eclipsing M-dwarf systems with precise mass and radius measurements. I also demonstrate the additional insight that can be gained into the formation and evolution of eclipsing binaries by finding certain seemingly rare types of orbital architectures as in these two systems.

In the third science chapter I present the discovery of the transiting brown dwarf NGTS-19b. This is a system below the hydrogen burning limit, only the 29th transiting brown dwarf known. These systems are rare and present many parallels with the M-dwarf binaries analysed as part of this work. With so few known this system is an important addition to the dearth of objects occupying the so called brown dwarf desert.

In the final science chapter I describe a new spectroscopic survey of M-dwarf binaries with the SpUpNIC spectrograph. With this survey we are able to observe a large number of systems and measure their masses to  with a precision of between 5-10% in order to populate the M-dwarf mass-radius parameter space, which can help to constrain stellar evolutionary models of these stars.