・石澤祐弥「Uranian satellite formation from a circumplanetary disk generated by a giant impact」（2020年度）
Uranus has a large axial tilt ∼ 90◦ and its regular satellites are orbiting almost on the equatorial plane of Uranus and have nearly circular orbits. As one of the most favorable candidate for the origin of these satellites, there is a giant impact scenario. A giant impact is a collision between proto-planets in the early stage of the formation of the Solar system. A giant impact scenario is the process that ejected materials around the planet through a giant impact is accumulated by self-gravity and eventually become satellites. This scenario is possible to explain the both origin of the large axial tilt and the regular satellites of Uranus. A giant impact is usually investigated by using the smoothed hydrodynamics (SPH) method. The impact simulations for Uranus were performed so far and they shows that a giant impact could explain the large axial tilt of Uranus. They also found that sufficient amount of materials to form the Uranian satellites could be ejected around Uranus. However, whether the Uranian satellites can actually form from such a circumplanetary disk remains unclear.
Here, we investigated the process of the satellite formation from a disk generated by a giant impact, using gravitational N-body simulation, which describes accumu- lation of solid particles under self-gravity, in order to explain the mass-orbit distri- bution of the current satellites of Uranus. we developed N-body simulation code for satellite formation, including gravitational interaction, collision, and merger between particles.
In Chapter 2 (Ishizawa et al., 2019), we modeled a debris disk of solids with several initial conditions inferred from the hydrodynamic simulations and performed N-body simulations to investigate in-situ satellite formation from the debris disk. We found that, in any case, the orbital distribution of the five major satellites could not be reproduced from the disk as long as the power index of its surface density is less than ∼2, which is similar value to that of the disk generated just after the giant impact. The satellites in the middle region obtained much larger masses than Ariel or Umbriel, while the outermost satellites did not grow to the mass of Oberon. Our results indicate that it is necessary to consider the thermal and viscous evolution of the evaporated disk after the giant impact to form the five major satellites through giant impact scenario.
In Chapter 3 (Ida et al., 2020), we show, by means of a theoretical model, that the Uranian satellite formation is regulated by the evolution of the impact-generated disk. We predict that the disk lost a substantial amount of water vapor mass and spread to the levels of the current system until the disk cooled down enough for ice condensation and accretion of icy particles to begin. From the predicted distribution of condensed ices, we found that the circumplanetary disk of solid ice could finally have a positive gradient in radial direction. We performed the N-body simulation for it and found that it is able to reproduce the observed mass-orbit configuration of the Uranian satellites.