  The core-accretion mechanism for gas giant formation may be too slow to
create all observed gas giant planets during reasonable gas disk lifetimes, but
it has yet to be firmly established that the disk instability model can produce
permanent bound gaseous protoplanets under realistic conditions. Based on our
recent simulations of gravitational instabilities in disks around young stars,
we suggest that, even if instabilities due to disk self-gravity do not produce
gaseous protoplanets directly, they may create persistent dense rings that are
conducive to accelerated growth of gas giants through core accretion. The rings
occur at and near the boundary between stable and unstable regions of the disk
and appear to be produced by resonances with discrete spiral modes on the
unstable side.
