THE FATE OF PLANETESIMALS IN TURBULENT DISKS WITH DEAD ZONES. II. LIMITS ON THE VIABILITY OF RUNAWAY ACCRETION

A critical phase in the standard model for planet formation is the runaway growth (RG) phase. During RG bodies in the 0.1-100 km size range (planetesimals) quickly produce a number of much larger seeds. The RG 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 RG, which is that the magnitude of the planetesimals' random (eccentric) motions is 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 magneto-rotational instability turbulence on the viability of the RG 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 mu m 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 RG. 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 similar to 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.