The Evolution of an Impact‐generated Partially Vaporized Circumplanetary Disk

We have performed a two-phase fluid numerical simulation of the evolution of a partially vaporized circumplanetary disk. In the giant-impact hypothesis, gravitational instability is crucial for the moon-forming process. It causes outward transportation of disk material beyond the Roche radius, where the disk material can accrete into a moon. There are two possible modes of gravitational instability: one is the instability of the melt-vapor mixture following disk cooling, and the other is the instability of the equatorial melt layer formed by sedimentation in the disk. In this paper, we examine the latter possibility, which has been poorly understood. The major results obtained are as follows: (1) The sedimentation of melt droplets occurs in about 10-2 yr, which is much faster than the disk cooling (~100 yr). (2) The gravitational instability of the melt layer occurs when the gas fraction is below the critical value (~0.7). When the gas fraction exceeds the critical value, neither type of gravitational instability occurs. These results suggest that the constituent of the melt layer is transported beyond the Roche radius much faster than the disk cooling unless the gas fraction is extremely high. As a consequence, a volatile-poor moon is expected to be formed while the disk still remains hot.