A “no-drift” runaway pile-up of pebbles in protoplanetary disks

Abstract

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 (αacc) and vertical stirring of pebbles in the disk midplane (αmid). In this study, the pebble-to-gas mass flux (Fp/g) was fixed as a parameter. Results. We find that, for a constant Fp/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 Fp/g ≳ 0.1 for more than ~10–100 kyr with nominal parameters: a gas mass flux of ≳10−8 M⊙ yr−1, τs ≃ 0.01−0.1, αmid ≲ 10−4, and αacc ≃ 10−3−10−2 at r ≲ 10 au, where r, τs, αmid, and αacc 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.