The Fate of Planetesimals in Turbulent Disks with Dead Zones. I. The Turbulent Stirring Recipe

TL;DR

This paper develops and tests simple scaling relations for how turbulence driven by magnetorotational instability stirs planetesimals in protoplanetary disks, accounting for Ohmic resistivity effects, and validates these relations with recent simulations.

Contribution

It introduces a parameter-independent set of scaling relations for planetesimal stirring in MRI-driven turbulence, incorporating Ohmic resistivity effects, and updates the density fluctuation saturation predictor.

Findings

Scaling relations successfully explain simulation results.

Stirring rate depends on disk parameters like gas density and magnetic flux.

Updated saturation predictor improves turbulence density fluctuation estimates.

Abstract

Turbulence in protoplanetary disks affects planet formation in many ways. While small dust particles are mainly affected by the aerodynamical coupling with turbulent gas velocity fields, planetesimals and larger bodies are more affected by gravitational interaction with gas density fluctuations. For the latter process, a number of numerical simulations have been performed in recent years, but a fully parameter-independent understanding has not been yet established. In this study, we present simple scaling relations for the planetesimal stirring rate in turbulence driven by magnetorotational instability (MRI), taking into account the stabilization of MRI due to Ohmic resistivity. We begin with order-of-magnitude estimates of the turbulence-induced gravitational force acting on solid bodies and associated diffusion coefficients for their orbital elements. We then test the predicted scaling relations using the results of recent Ohmic-resistive MHD simulations by Gressel et al. We find that these relations successfully explain the simulation results if we properly fix order-of-unity uncertainties within the estimates. We also update the saturation predictor for the density fluctuation amplitude in MRI-driven turbulence originally proposed by Okuzumi & Hirose. Combination of the scaling relations and saturation predictor allows to know how the turbulent stirring rate of planetesimals depends on disk parameters such as the gas column density, distance from the central star, vertical resistivity distribution, and net vertical magnetic flux. In Paper II, we apply our recipe to planetesimal accretion to discuss its viability in turbulent disks.