Discs around giant protoplanets

This thesis contains a study of circumplanetary discs (CPDs) around giant protoplanets. They are modelled with three-dimensional, hydrodynamical multifluid simulations, with the aim of understanding their dust grain size distributions and thus opacities and dust masses.

By comparing 2-fluid (gas + 1 dust grain size) and multifluid (gas + multiple grain size) simulations for a 1 M_Jup protoplanet at 10 AU, it is ascertained that every dust grain size accretes onto the CPD with the same efficiency as if it and the gas were a 2-fluid system. Dust grains of size 1um-100um accrete with comparable efficiency. 1mm dust grains are blocked at the outer gap edge or taken into the horseshoe region, where they reach high concentration. They are extremely inefficient at accreting onto the CPD. This leads to low CPD dust-to-gas ratio ~ 8 × 10^-4.

By comparing 9 multifluid simulations of 10 M_Earth, 100 M_Earth and 1000 M_Earth protoplanets at 5 AU, 15 AU and 30 AU, it is demonstrated that the thermal criterion R_Hill > H accurately predicts which can form gaps and CPDs and which only envelopes. The crucial governing parameter is shown to be a_dec the grain size at which accretion efficiency decreases. Small a_dec means low CPD dust mass, because most dust mass is in large grains. A parametrisation with a_dec is an excellent fit to the grain size distribution. Knowing a_dec gives that distribution, thus giving opacity, to translate observed fluxes into masses.

a_dec falls as semimajor axis rises. High protoplanet mass also makes a_dec smaller because of a deeper gap. Therefore CPD dust mass sometimes falls as protoplanet mass rises.

The results suggest that massive giant planets at >~ 30 AU will have extremely low dust-to-gas mass ratios (~ 2 × 10^-4). They will be unable to form rocky satellites and will be very poor in Fe and silicates and rich in H/He.