New Growth Mechanism of Dust Grains in Protoplanetary Disks with Magnetically Driven Disk Winds

Abstract

We discovered a new growth mode of dust grains to kilometer-size bodies in protoplanetary disks that evolve via viscous accretion and magnetically driven disk winds (MDWs). We solved an approximate coagulation equation of dust grains with time-evolving disks that consist of both gas and solid components using a one-dimensional model. With grain growth, all solid particles initially drift inward toward the central star due to the gas drag force. However, the radial profile of gas pressure, P, is modified by the MDW that disperses the gas in an inside-out manner. Consequently, a local concentration of solid particles is created by the converging radial flux of drifting dust grains at the location with a convex-upward profile of P. When the dimensionless stopping time, St, exceeds unity there, the solid particles spontaneously reach the growth-dominated state because of the positive feedback between the suppressed radial drift and the enhanced accumulation of dust particles that drift from the outer part. Once the solid particles are in the drift-limited state, the above-mentioned condition of St for dust growth is equivalent to Σd/Σg ≳ η, where Σd/Σg is the dust-to-gas surface-density ratio and η is the dimensionless radial pressure-gradient force. As a consequence of the successful growth of dust grains, a ring-like structure containing planetesimal-size bodies is formed at the inner part of the protoplanetary disks. Such a ring-shaped concentration of planetesimals is expected to play a vital role in the subsequent planet formation.

Distant Formation and Differentiation of Outer Main Belt Asteroids and Carbonaceous Chondrite Parent Bodies

“…We suggest that there might be multiple pressure traps to separate reservoirs. Possible origins of pressure traps include gas‐dust viscous gravitational instability (Tominaga et al., 2019), magneto‐hydrodynamical wind (Taki et al., 2021), sublimation near the snow line(s) (Charnoz et al., 2021), and disk–planet interaction (Kanagawa et al., 2015). Such multiple pressure traps in a single protoplanetary disk are thought to create ringed structures which were found to be common in protoplanetary disks observed recently by the Atacama Large Millimeter/submillimeter Array (the spacial scales of rings are several tens of au, Andrews et al., 2018; Dullemond et al., 2018).…”

Coagulation Instability in Protoplanetary Disks: A Novel Mechanism Connecting Collisional Growth and Hydrodynamical Clumping of Dust Particles

“…The moment equation for dust coagulation we adopted was derived in Sato et al (2016) from the vertically integrated Smoluchowski equation (see also Taki et al 2021). In this appendix, we review the derivation and mention its validity for the analysis on coagulation instability.…”

Distant formation and differentiation of outer main belt asteroids and carbonaceous chondrite parent bodies

“…We suggest that there might be multiple pressure traps to separate reservoirs. Possible origins of pressure traps include gas-dust viscous gravitational instability (Tominaga et al, 2019), magneto-hydrodynamical wind (Taki et al, 2021), sublimation near the snow line(s) (Charnoz et al, 2021), and disk-planet interaction (Kanagawa et al, 2015). Such multiple pressure traps in a single protoplanetary disk are thought to create ringed structures which were found to be common in protoplanetary disks observed recently by the Atacama Large Millimeter/submillimeter Array (the spacial scales of rings are several tens of au, Andrews et al, 2018;Dullemond et al, 2018).…”