  Sufficiently massive growing giant planets have circumplanetary disks, and
the capture of solid bodies by the disks would likely influence the growth of
the planets and formation of satellite systems around them. In addition to dust
particles that are supplied to the disk with inflowing gas, recent studies
suggest the importance of capture of planetesimals whose motion is decoupled
from the gas, but orbital evolution of captured bodies in circumplanetary gas
disks has not been studied in detail. In the present work, using three-body
orbital integration and analytic calculations, we examine orbital
characteristics and subsequent dynamical evolution of planetesimals captured by
gas drag from circumplanetary gas disks. We find that the semi-major axes of
the planet-centered orbits of planetesimals at the time of permanent capture
are smaller than about one third of the planet's Hill radius in most cases.
Typically, captured bodies rapidly spiral into the planet, and the rate of the
orbital decay is faster for the retrograde orbits due to the strong headwind
from the circumplanetary gas. When a planetesimal captured into a retrograde
orbit suffers from sufficiently strong gas drag before spiraling into the
planet, its orbit turns to the prograde direction at a radial location that can
be explained using the Stokes number. We also find that those captured into
certain types of orbits can survive for a long period of time even under gas
drag both in the prograde and retrograde cases, which may be important for the
origin of irregular satellites of giant planets.
