The Dynamics of Discs in Young Stellar Systems

Star formation produces single, binary, and higher order young stellar systems that can host protostellar discs, as evidenced by recent observations of embedded disc structure. In this thesis, I present numerical simulations of circumsingle and circumbinary discs in this context.

At early times, embedded discs are likely massive and potentially gravitationally-unstable. Disc temperature (among other parameters) controls the stability of discs - cooling discs faster than they can be heated can result in disc fragmentation, but otherwise can produce long-lasting spiral structure. I study circumsingle discs in both regimes.

I investigate the critical cooling timescale convergence using the established βc parameterisation, finding convergence only for constant linear artificial viscosity with resolution. There is no physical convergence for various artificial viscosity settings or modified disc models that boost resolution. Suitable parameter choices may produce convergence but at significant computational cost (i.e. high resolutions), contradicting the appeal of βc.

I model fragmentation-stable, self-gravitating discs that grow from low-mass, making comparisons with observations. I find these discs agree with standard disc models and the observed disc structure of Elias 2-27, with no dependency on growth rate. This suggests discs naturally approach a quasi-steady state of marginal stability. However, I expect disc evolution to diverge for growth rates exceeding the local transport rate (i.e. fragmentation-unstable discs).

Like self-gravitating discs, prograde circumbinary discs are also perturbed, here from resonances between the binary and disc orbits. Waves are launched in the disc allowing the binary to transfer its angular momentum to the disc, acting against the viscous torque. I study this disc-binary interaction by varying disc thickness, viscosity and equation of state, showing how the boundary between binary orbit expansion and contraction changes. For viscous discs, the boundary is found at H/R = 0.1−0.2, though coalescence is seen for low-viscosity, thick discs. Introducing a stratified thermal structure slows binary orbit evolution.