Ultracool Benchmark Companions to Gaia Stars

Ultracool dwarfs are stellar and sub-stellar objects with spectral types of M7 or later and effective temperatures below 2700K. Over 1300 have been discovered to date, both as free-floating objects and companions to more massive stars. It is difficult to constrain the physical properties of an ultracool dwarf, as they have low luminosities and are generally not found as close companions. Models for the evolution, composition and atmospheres of ultracool dwarfs depend on having examples with independently measured physical properties; these are known as `benchmark' ultracool dwarfs. To independently determine a physical property, the ultracool dwarf must be associated with a star. This thesis describes a search for ultracool companions that may be suitable benchmarks using various optical and infrared surveys in combination with the Gaia TGAS and DR2 catalogues. This search utilises existing methods in novel ways to identify and select candidates, with a particular focus on identifying rare or unusual objects that may have features for which there is currently no benchmark. This thesis presents 32 candidate companions that are recommended for follow-up observations, plus a further 227 candidates that require further analysis in future work.