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

TL;DR

This paper investigates how turbulence in protoplanetary disks affects the early runaway growth of planetesimals, finding that in outer disk regions, turbulence inhibits planetesimal formation, challenging classical planet formation models.

Contribution

It introduces a semi-analytical framework to assess turbulence effects on planetesimal growth, highlighting the limits of runaway growth in outer disk regions.

Findings

Runaway growth is suppressed beyond 5 AU in turbulent disks.

Turbulence reduces dust surface area significantly, affecting dust dynamics.

Classical planetesimal-driven planet formation is unlikely in outer turbulent disk regions.

Abstract

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.