The role of massive protoplanetary discs in the formation of stars and planets

It is now well known that stars form inside Giant Molecular Clouds (GMCs) made up primarily of cold, dense molecular hydrogen and dust. Although the molecular cloud cores from which stars form rotate very slowly, they still contain far more angular momentum than any single star. It is now well understood that most of the mass that ultimately forms a star - in particular low-mass stars - must first pass through a flattened circumstellar disc. What is still unknown is how the angular momentum is transported outwards through the disc, allowing mass to move inwards onto the star. Although, it has been suggested the magnetohydrodynamic (MHD) turbulence may provide a mechanism for transporting angular momentum, it seems unlikely that these discs are sufficiently ionised for this to operate effectively. Since most of the mass that forms the central star must first pass through the disc, it is quite likely that - comared to the mass of the central star - these discs may be relatively massive. This suggests that these discs could be susceptible to the growth of a gravitational instability which will lead to the formation of spiral density waves - a process analagous to the formation of spiral arms in disc galaxies. These spiral density waves act to transport angular momentum outwards allowing mass to accrete onto the central star. A primary goal of the work here will be to investigate - using analytic calculations and numerical simulations - the evolution of massive discs around young stars with the aim of understanding if this gravitational instability can be the primary transport mechanism in young stellar discs. In particular, these models will include detailed thermodynamics and radiation transfer as it is now clear that the evolution of the gravitationally instability depends strongly on the heating and cooling processes in the disc. If the gravitational instability is important, it is likely to be so during the earliest stages of star formation, which is also the period during which dust grains are expected to grow to form planetesimals - the building blocks of planets. One aspect of the planet formation process that is still not understood is how cm-sized particles grow quickly into kilometre-sized bodies. It has already been suggested that spitral density waves may play a role in planet formation by producing dense collections of particles that can either grow rapidly through collisions or could grow directly through gravitational collapse. Together with studying the evolution of gravitationally unstable discs we will also investigate if the resulting spiral density waves can play a role in the formation of planetesimals. This will extend earlier work in that we will have a detailed understanding of where spiral waves are likely to exist in protostellar discs and how strong they are likely to be. We will also aim to quantify the influence of spiral density waves on the population of cm-sized grains to see if their subsequent growth occurs through collisions or if they can become sufficiently dense to grow through direct gravitational collapse.