Characterising the gas and dust in protoplanetary discs around Herbig stars

This thesis contains a study of the circumstellar discs around intermediate mass stars. Through observational data and protoplanetary disc modelling, the physical structure and composition of protoplanetary discs are investigated with regards to their capacity for planet formation. In-depth analysis of millimetre-wavelength interferometric observations are carried out on the circumstellar environment of two Herbig stars. Firstly, the distribution of gas and dust in the gas-rich, potentially planet-hosting disc of HD100546 is characterised. Using ALMA observations of 1.3mm continuum and CO isotopologues tracing the disc midplane, estimates of disc mass are calculated, constraints on the size of dust grains inferred and evidence for midplane counterparts to scattered light features are identified. Secondly, an analysis of the more evolved circumstellar disc around Herbig star HD141569 is made in order to investigate the mass content of the disc and inform the debate as to its evolutionary stage. New ALMA observations presented in this thesis and new midplane structures in the gas and dust that support an intermediary stage of evolution between the protoplanetary disc and debris disc regimes. Finally, modelling of the pre-main sequence evolution of stars across the stellar mass range at which exoplanet detections peak is combined with Monte Carlo radiative transfer and modelling of the evolution of midplane gas and dust in order to study the impact of stellar evolution on the midplanes of protoplanetary discs. Variations in midplane temperature profles result in different locations of key snowlines in the disc, which in turn produces variations in the molecular composition of the local disc. The results quantify how snowline locations depend on stellar luminosity evolution. This modelling procedure is applied to the system of HR8799 in order to put constraints on the time and location within the disc at which wide-orbit planets could have formed based on their atmospheric C/O ratio. The results support an early formation time, within around 1 Myr, for the carbon-rich exoplanet HR8799b.