Signatures of Companions in Transitional Discs: Spiral Arms, Cavities, and Dust Asymmetries

Modern observations of protoplanetary discs are revealing a multitude of substructures in these systems, such as spiral arms, cavities, gaps, rings, and large scale asymmetries, in both the gas and dust distribution. It has long been thought that such substructures can arise due companion-disc interactions. However, confirming the existence of these companions by direct methods remains challenging. Therefore, a common method of studying the properties of these companions is through a combination of hydrodynamical simulations and radiative transfer modelling. One particular class of protoplanetary discs, known as transitional discs, are a particularly promising site for testing planet formation and planet-disc interaction theories. These discs exhibit large, cleared out cavities, which are proposed to arise due to dynamical clearing by planetary mass companions. As their name suggests, they are thought to be a transitional phase between gas rich protoplanetary discs, and gas depleted debris discs. In this thesis, I study the nature of the companions in transitional discs through the use of hydrodynamical simulations and radiative transfer modelling. I match multi-wavelength observations of these systems, ranging from near infrared scattered light to centimetre thermal emission. My work challenges some of the prominent interpretations of the structures arising in transitional discs. I show that stellar mass companions may present a more compelling explanation for the structures inside some transitional discs, such as IRS 48 and AB Aurigae. I argue that these are in fact circumbinary discs, discs which orbit around two stars, rather than transitional discs.I also show that planetary mass companions on eccentric orbits inside of the cavity of transition discs may be giving rise to the spectacular spiral arm morphology seen in the transitional disc MWC 758. This is in strong contrast to the current favoured hypothesis for the origin of these spiral arms, where the companion is external to the spiral arms.The results of my thesis imply that the population of companions in transitional discs is more eclectic than previously thought. I postulate that many of the well studied transitional discs of the last decade or so are in fact circumbinary discs. This implies that as a class, transitional discs are composed of distinct two populations; circumbinary discs, and planet hosting transitional discs. The kinematics of the disc are an important diagnostic for differentiating between these two sub-populations.