  In order to understand the physical processes underlying clump formation in
outflow from supercritical accretion flow, we performed two-dimensional
radiation hydrodynamic (RHD) simulations. We focus our discussion on the nature
of RHD instability in marginally optically thick, plane-parallel,
super-Eddington atmosphere. Initially we set two-layered atmosphere with a
density contrast of 100 exposed to strong, upward continuum-radiation force;
the lower layer is denser than the upper one, condition for an RHD instability.
We assume non-zero but negligible gravitational force, compared with the
radiation force. We find that short wavelength perturbations first grow,
followed by growth of longer wavelength patterns, which lead to the formation
of clumpy structure. The typical size of clumps (clouds) corresponds to about
one optical depth. An anti-correlation between the radiation pressure and the
gas pressure is confirmed: this anti-correlation provides a damping mechanism
of longer wavelength perturbations than the typical clump size. Matter and
radiation energy densities are correlated. These features are exactly what we
found in the radiation-magnetohydrodynamic (radiation-MHD) simulations of
supercritical outflow.
