  We investigate gas accretion flow onto a circumplanetary disk from a
protoplanetary disk in detail by using high-resolution three-dimensional
nested-grid hydrodynamic simulations, in order to provide a basis of formation
processes of satellites around giant planets. Based on detailed analyses of gas
accretion flow, we find that most of gas accretion onto circumplanetary disks
occurs nearly vertically toward the disk surface from high altitude, which
generates a shock surface at several scale heights of the circumplanetary disk.
The gas that has passed through the shock surface moves inward because its
specific angular momentum is smaller than that of the local Keplerian rotation,
while gas near the midplane in the protoplanetary disk cannot accrete to the
circumplanetary disk. Gas near the midplane within the planet's Hill sphere
spirals outward and escapes from the Hill sphere through the two Lagrangian
points L$_1$ and L$_2$. We also analyze fluxes of accreting mass and angular
momentum in detail and find that the distributions of the fluxes onto the disk
surface are well described by power-law functions and that a large fraction of
gas accretion occurs at the outer region of the disk, i.e., at about 0.1 times
the Hill radius. The nature of power-law functions indicates that, other than
the outer edge, there is no specific radius where gas accretion is
concentrated. These source functions of mass and angular momentum in the
circumplanetary disk would provide us with useful constraints on the structure
and evolution of the circumplanetary disk, which is important for satellite
formation.
