Gaps, rings, and spirals in scattered light: Discs observed around Herbig Ae/Be and T-Tauri stars

Planetary systems have their origins in protoplanetary discs, which are thin, rotating struc- tures of dust and gas encircling protostellar objects. The processes governing the formation of protoplanets are expected to influence the structure of these discs. The resulting structures around other stars can now be observed in scattered light, using instruments such as the Gemini Planet Imager (GPI) with techniques including Extreme Adaptive Optics and Polarimetric Differential Imaging. The observations presented in this thesis were obtained with GPI for Gemini-LIGHTS, a survey aiming to categorise the structures seen around a representative sample of Herbig Ae/Be and T-Tauri objects. Early Gemini-LIGHTS data showed irregular structures that eluded simple classifications. I present a detailed analysis of five of these objects, which include new detections of four objects and an improved-quality image of a fifth, FU Ori, which revealed a previously-unknown shadow. I discuss these objects in the context of age and misaligned discs. I present observations of the Herbig B[e] star HD 45677, which has had a highly-contested evolutionary status for decades. I created a Monte Carlo Radiative Transfer (MCRT) model of HD 45677 that was consistent with three data sets: a small-scale inner wall; a large-scale envelope with bipolar cavity; and ultraviolet to far-infrared photometry. This analysis strongly supports the classification of HD 45677 as a pre-main-sequence object. Finally, the transition disc around HD 34700 showed the highest number of spiral arms ever observed, each of which was unexpectedly loosely-wound. I developed a method to quantitatively characterise the spiral winding properties. I found that there are at least eight arms, each with pitch angle >30◦, and that hyperbolic spirals fit the most-constrained structure best but cannot describe the whole system. These findings are incompatible with current spiral formation theories, and so provide a starting point for follow-up observations and modelling.Science and Technology Facilities Counci