A numerical study of the gravitational instability in protostellar discs

Abstract

Young protostellar discs may be massive enough that they become unstable and fragment, leading to the formation of planets, brown dwarfs, and low-mass stars. This thesis examines the development of the gravitational instability in discs: how it occurs and whether planets can form through gravitational fragmentation.
I consider the effects of radiative feedback by objects formed through gravitational fragmentation in discs, and compare simulations which consider three cases of radiative feedback: none, continuous, and episodic. I find that when radiative feedback is continuous, only one object forms as its radiative feedback supresses further fragmentation. However, when radiative feedback is episodic, further fragmentation occurs as the disc cools between episodes. Generally, the stronger the radiative feedback, the fewer objects ultimately form through gravitational fragmentation. Since multiple objects are formed, those of lower mass are ejected due to gravitational interactions. On the other hand, the more massive objects accrete a sufficient amount of gas to eventually become brown dwarfs or low-mass stars. Therefore, disc fragmentation may be a significant source of free-floating planets and companion brown dwarfs.
I study the effects of gravitational instability in protostellar discs around M-dwarfs and determine the minimum disc mass required for fragmentation. Disc-to-star mass ratios of between q ~ 0.3 and q ~ 0.6 are found to be necessary, and although the metallicity of the disc does not affect this, a high metallicity can inhibit fragmentation altogether. The gravitational fragmentation of protostellar discs around M-dwarfs results in the formation of massive protoplanets: they have initial masses above 5 Mjup and for on wide orbits (~ 10 – 100 AU from the host star). Therefore, the massive planets on wide orbits observed around M-dwarfs may have formed via gravitational fragmentation, provided that they were attended by relatively massive discs during their early phase of formation.