A "no-drift" runaway pile-up of pebbles in protoplanetary disks. II. Characteristics of the resulting planetesimal belt

Description

Context. Forming planetesimals from pebbles is a major challenge in our current understanding of planet formation. In a protoplanetary disk, pebbles drift inward near the disk midplane via gas drag and they may enter a zone of reduced turbulence (dead zone). In this context, we identified that the backreaction of the drag of pebbles onto the gas could lead to a runaway pile-up of pebbles, the so-called no-drift mechanism for the formation of planetesimals.
Aims: We improve upon the previous study of planetesimal formation from accumulating pebbles via the no-drift mechanism by investigating the nature and characteristics of the resultant planetesimal belt.
Methods: We performed 1D diffusion-advection simulations of drifting pebbles in the outer region of a modeled dead zone by including a pebble-gas backreaction to the radial drift of pebbles and including planetesimal formation via the streaming instability. We independently considered the parameters that regulate gas accretion (α_acc) and vertical stirring of pebbles in the disk midplane (α_mid). In this study, the pebble-to-gas mass flux (F_p/g) was fixed as a parameter.
Results: We find that, for a constant F_p/g, after the criteria of the no-drift mechanism are satisfied, planetesimals initially form within a narrow ring whose width expands as accumulating pebbles radially diffuse over time. The system finally reaches a steady-state where the width of the planetesimal belt no longer changes, followed by a continuous formation of planetesimals. A non-negligible total mass of planetesimals (more than one Earth mass) is formed via the no-drift mechanism for a disk having F_p/g ≳ 0.1 for more than ~10-100 kyr with nominal parameters: a gas mass flux of ≳10⁻⁸ M_⊙ yr⁻¹, τ_s ≃ 0.01−0.1, α_mid ≲ 10⁻⁴, and α_acc ≃ 10⁻³−10⁻² at r ≲ 10 au, where r, τ_s, α_mid, and α_acc are the heliocentric distance, the Stokes number, and the parameters in a dead zone controlling the efficiencies of vertical turbulent diffusion of pebbles (i.e., scale height of pebbles) and gas accretion of the α-disk (i.e., gas surface density), respectively.

Abstract

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