  The growth of small planetesimals into large planetary embryos occurs much
before the dispersal of the gas from the protoplanetary disk. The planetesimal
- gaseous-disk interactions give rise to migration and orbital evolution of the
planetesimals/planets. Small planetesimals are dominated by aerodynamic gas
drag. Large protoplanets, $m\sim0.1M_{\oplus}$, are dominated by type I
migration \emph{differential} torque. There is an additional mass range,
$m\sim10^{21}-10^{25}g$ of \emph{intermediate mass} planetesimals (IMPs), where
gravitational interactions with the disk dominate over aerodynamic gas drag,
but for which such interactions were typically neglected. Here we model these
interactions using the \emph{gas dynamical friction} (GDF) approach, previously
used to study the disk-planet interactions at the planetary mass range. We find
the critical size where GDF dominates over gas drag, and then study the
implications of GDF on single IMPs. We find that planetesimals with small
inclinations rapidly become co-planar. Eccentric orbits circularize within a
few Myrs, provided the the planetesimal mass is large, $m\gtrsim10^{23}g$ and
that the initial eccentricity is low, $e\lesssim0.1$. Planetesimals of higher
masses, $m\sim10^{24}-10^{25}g$ inspiral on a time-scale of a few Myrs, leading
to \emph{an embryonic migration} to the inner disk. This can lead to an
over-abundance of rocky material (in the form of IMPs) in the inner
protoplanetary disk ($<1$AU) and induce rapid planetary growth. This can
explain the origin of super-Earth planets. In addition, GDF damps the
velocities of IMPs, thereby cooling the planetesimal disk and affecting its
collisional evolution through quenching the effects of viscous stirring by the
large bodies.
