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.