We investigate the accretion of angular momentum onto a protoplanet system
using three-dimensional hydrodynamical simulations. We consider a local region
around a protoplanet in a protoplanetary disk with sufficient spatial
resolution. We describe the structure of the gas flow onto and around the
protoplanet in detail. We find that the gas flows onto the protoplanet system
in the vertical direction crossing the shock front near the Hill radius of the
protoplanet, which is qualitatively different from the picture established by
two-dimensional simulations. The specific angular momentum of the gas accreted
by the protoplanet system increases with the protoplanet mass. At Jovian orbit,
when the protoplanet mass M_p is M_p < 1 M_J, where M_J is Jovian mass, the
specific angular momentum increases as j \propto M_p. On the other hand, it
increases as j \propto M_p^2/3 when the protoplanet mass is M_p > 1 M_J. The
stronger dependence of the specific angular momentum on the protoplanet mass
for M_p < 1 M_J is due to thermal pressure of the gas. The estimated total
angular momentum of a system of a gas giant planet and a circumplanetary disk
is two-orders of magnitude larger than those of the present gas giant planets
in the solar system. A large fraction of the total angular momentum contributes
to the formation of the circumplanetary disk. We also discuss the satellite
formation from the circumplanetary disk.Comment: 39 pages,13 figures, Submitted to ApJ, For high resolution figures
see http://www2.scphys.kyoto-u.ac.jp/~machidam/jupiter2/ms08jan22.pd