  Recent progress in transit photometry opened a new window to the interior of
super-Earths. From measured radii and masses, we can infer planetary internal
compositions. It has been recently revealed that super-Earths are diverse in
composition. Such a diversity is thought to arise from diversity in volatile
content. The stability of the volatile components is to be examined, because
hot super-Earths undergo photo-evaporative mass loss. While several studies
investigated the impact of photo-evaporative mass loss on hydrogen-helium
envelopes, there are few studies as to the impact on water-vapor envelopes. To
obtain theoretical prediction to future observations, we also investigate the
relationships among masses, radii, and semimajor axes of water-rich
sub/super-Earths that have undergone photo-evaporative mass loss. We simulate
the interior structure and evolution of sub/super-Earths that consist of a
rocky core surrounded by a water envelope, including mass loss due to the
stellar XUV-driven energy-limited hydrodynamic escape. We find that the
photo-evaporative mass loss has a significant impact on the evolution of hot
sub/super-Earths. We then derive the threshold planetary mass and radius below
which the planet loses its water envelope completely as a function of the
initial water content, and find that there are minimums of the threshold mass
and radius. We constrain the domain in the parameter space of planetary mass,
radius, and semimajor axis in which sub/super-Earths never retain water
envelopes in 1-10 Gyr. This would provide an essential piece of information for
understanding the origin of close-in low-mass planets. The current
uncertainties in stellar XUV flux and its heating efficiency, however, prevent
us from deriving robust conclusions. Nevertheless, it seems to be a robust
conclusion that Kepler planet candidates contain a significant number of rocky
sub/super-Earths.
