  Motivated by recent discoveries of low-density super-Earths with short
orbital periods, we have investigated in-situ accretion of H-He atmospheres on
rocky bodies embedded in dissipating warm disks, by simulating quasi-static
evolution of atmospheres that connect to the ambient disk. We have found that
the atmospheric evolution has two distinctly different outcomes, depending on
the rocky body's mass: While the atmospheres on massive rocky bodies undergo
runaway disk-gas accretion, those on light rocky bodies undergo significant
erosion during disk dispersal. In the atmospheric erosion, the heat content of
the rocky body that was previously neglected plays an important role. We have
also realized that the atmospheric mass is rather sensitive to disk temperature
in the mass range of interest in this study. Our theory is applied to
recently-detected super-Earths orbiting Kepler-11 to examine the possibility
that the planets are rock-dominated ones with relatively thick H-He
atmospheres. The application suggests that the in-situ formation of the
relatively thick H-He atmospheres inferred by structure modeling is possible
only under restricted conditions; namely, relatively slow disk dissipation
and/or cool environments. This study demonstrates that low-density super-Earths
provide important clues to understanding of planetary accretion and disk
evolution.
