Brown dwarfs and giant exoplanets: bridging observations and theory with statistical methods
Fontanive, Clémence Cécile Pia
Stellar physics has been widely studied over the last century, with theoretical models for stars robustly tested by decades of observations. In contrast, studies of brown dwarfs and extra-solar giant planets were only possible for the last two decades due to the intrinsically faint luminosity of these objects. As a result, the fundamental properties of substellar objects are still poorly constrained. Formation mechanisms for brown dwarfs and planetary-mass objects remain heavily debated, and atmospheric models widely lack empirical validation at the lowest masses and temperatures. Theoretical models are currently the only available way to infer physical parameters (e.g. mass, temperature) for isolated objects and directly-imaged companions on wide orbits, and are thus widely used by the community in spite of the extremely high uncertainties they carry. More stringent observational constraints, or new alternative methods, are essential to allow for a further and deeper understanding of brown dwarfs and giant planets. Robust population studies provide invaluable insights into formation processes and empirical trends. Statistical methodologies may thus be used to refine theoretical models and obtain a more complete overview of the properties and statistics of the substellar populations. This dissertation addresses three problems in the framework of brown dwarfs and giant exoplanets, namely, substellar binary properties, the formation of massive planets and brown dwarfs around stars, and the detection and model-independent masses of direct imaging systems. Chapter 2 presents results from a multiplicity survey investigating the binary statistics of the lowest-mass brown dwarfs. As binarity is a direct outcome of formation, observed trends as a function of mass provide valuable insights into formation processes. In Chapter 3, I conduct a search for stellar companions to stars with close-in, massive planets, as a test of formation theory for giant planets and brown dwarfs on small orbital separations. Chapter 4 introduces a dedicated tool designed to identify new wide-orbit companions and constrain the orbits of astrometric systems. The method allows for the determination of dynamical masses for directly-imaged companions, a powerful way to circumvent the large uncertainties introduced by models. The common goal to these projects is to infer new, crucial observational constraints for formation theories or atmospheric models in the substellar regime. This will in turn provide a more comprehensive view of the characteristics and demographics of brown dwarfs and exoplanets.