  A critical phase in the standard model for planet formation is the runaway
growth phase. During runaway growth bodies in the 0.1--100 km size range
(planetesimals) quickly produce a number of much larger seeds. The runaway
growth phase is essential for planet formation as the emergent planetary
embryos can accrete the leftover planetesimals at large gravitational focusing
factors. However, torques resulting from turbulence-induced density
fluctuations may violate the criterion for the onset of runaway growth, which
is that the magnitude of the planetesimals' random (eccentric) motions are less
than their escape velocity. This condition represents a more stringent
constraint than the condition that planetesimals survive their mutual
collisions. To investigate the effects of MRI turbulence on the viability of
the runaway growth scenario, we apply our semi-analytical recipes of Paper I,
which we augment by a coagulation/fragmentation model for the dust component.
We find that the surface area-equivalent abundance of 0.1 micron particles is
reduced by factors 10^2--10^3, which tends to render the dust irrelevant to the
turbulence. We express the turbulent activity in the midplane regions in terms
of a size s_run above which planetesimals will experience runaway growth. We
find that s_run is mainly determined by the strength of the vertical net field
that threads the disks and the disk radius. At disk radii beyond 5 AU, s_run
becomes larger than ~100 km and the collision times among these bodies longer
than the duration of the nebula phase. Our findings imply that the classical,
planetesimal-dominated, model for planet formation is not viable in the outer
regions of a turbulent disk.
