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A critical phase in the standard model for planet formation is the runawaygrowth phase. During runaway growth bodies in the 0.1--100 km size range(planetesimals) quickly produce a number of much larger seeds. The runawaygrowth phase is essential for planet formation as the emergent planetaryembryos can accrete the leftover planetesimals at large gravitational focusingfactors. However, torques resulting from turbulence-induced densityfluctuations may violate the criterion for the onset of runaway growth, whichis that the magnitude of the planetesimals' random (eccentric) motions are lessthan their escape velocity. This condition represents a more stringentconstraint than the condition that planetesimals survive their mutualcollisions. To investigate the effects of MRI turbulence on the viability ofthe 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 isreduced by factors 10^2--10^3, which tends to render the dust irrelevant to theturbulence. We express the turbulent activity in the midplane regions in termsof a size s_run above which planetesimals will experience runaway growth. Wefind that s_run is mainly determined by the strength of the vertical net fieldthat threads the disks and the disk radius. At disk radii beyond 5 AU, s_runbecomes larger than ~100 km and the collision times among these bodies longerthan the duration of the nebula phase. Our findings imply that the classical,planetesimal-dominated, model for planet formation is not viable in the outerregions of a turbulent disk.

The fate of planetesimals in turbulent disks with dead zones. II. Limits on the viability of runaway accretion