Planetesimal Formation in Magnetorotationally Dead Zones: Critical Dependence on the Net Vertical Magnetic Flux

TL;DR

This paper investigates how the net vertical magnetic flux influences planetesimal formation in MRI-dead zones, highlighting the importance of low NVF for successful formation of icy and rocky planetesimals.

Contribution

It demonstrates that low net vertical magnetic flux is crucial for planetesimal formation in MRI-inactive zones, providing an analytic framework based on resistive MHD simulations.

Findings

Low NVF (<10 mG) enables icy planetesimal formation across fragmentation barriers.

Secular gravitational instability can form rocky planetesimals within similar NVF ranges.

The evolution of NVF during disk formation impacts planetesimal formation outcomes.

Abstract

Turbulence driven by magnetorotational instability (MRI) affects planetesimal formation by inducing diffusion and collisional fragmentation of dust particles. We examine conditions preferred for planetesimal formation in MRI-inactive "dead zones" using an analytic dead-zone model based on our recent resistive MHD simulations. We argue that successful planetesimal formation requires not only a sufficiently large dead zone (which can be produced by tiny dust grains) but also a sufficiently small net vertical magnetic flux (NVF). Although often ignored, the latter condition is indeed important since the NVF strength determines the saturation level of turbulence in MRI-active layers. We show that direct collisional formation of icy planetesimal across the fragmentation barrier is possible when the NVF strength is lower than 10 mG (for the minimum-mass solar nebula model). Formation of rocky planetesimals via the secular gravitational instability is also possible within a similar range of the NVF strength. Our results indicate that the fate of planet formation largely depends on how the NVF is radially transported in the initial disk formation and subsequent disk accretion processes.