The formation and evolution of discs around young stars

Abstract

In this thesis I use three-dimensional radiation hydrodynamic calculations to pro- vide an insight into the properties of protostellar discs, and how their environment can impact them. Through the use of large star-cluster formation calculations, I find some of the ways the metallicity of the molecular cloud can impact discs. The statistical proper- ties of discs formed in one of these calculations match those of observational surveys very well. Motivated by the ability of hydrodynamic calculations to make comparisons with observations I extend the existing numerical methods to model the dynamics of small dust grains in an astrophysical context. When considering the effects of metallicity on discs I find discs in low metallicity environments tend to be smaller and less well aligned with spin of their protostar. This is due to the low opacity caused by low metallicity and the increased cooling rates because of this. This leads to an enhanced rate of small scale fragmentation. The fragmentation causes an increase in stellar multiplicity and causes the truncation of disc radii. Using a star-cluster formation calculation at solar metallicity I study the occurrence rate of circumbinary discs. I find that for a given sample of binaries one would expect around 35 per cent of systems to host a circumbinary disc. However, circumbinary discs around binaries with a semi-major axis greater than 100au are predicted to be very rare. I compare the distribution of all known observed of mutual inclination angles between the circumbinary disc and its host binary orbit with those formed in the calculation. The underlying distributions of the two samples are in good agreement. I have developed an implicit algorithm to model the dynamics of small dust grains. I find this method to be fast, accurate, and avoids several issues there are present with the current dust-as-mixture method in smoothed particle hydrodynamics. This method is applied to dusty discs with an embedded planet. In these calculations, I confirm the discovery of a vertical dust transport mechanism that has only been recently identified by other authors.