Dust-gas coupling in turbulent and MHD-wind-driven protoplanetary disks: implications for the formation of rocky planets

Grid of Log₁₀(τ_P,PA/τ_P,PW ) at t = 0 for a Mars-sized embryo at (a) r = 1 AU and (b) r = 20 AU. In (a), the values ​​corresponding to disks with low ˜α and, to a lesser extent, high ψ are

The degree of coupling between dust particles and the surrounding gas in protoplanetary disks is quantified by the dimensionless Stokes number. The Stokes number (St) determines the particle size and spatial distribution and thus establishes the predominant mode of planetary accretion in different disk regions.

In this paper, we model the characteristic St value of particles over time in disks evolving under both turbulent viscosity and magnetohydrodynamic (MHD) disk winds. In both turbulence- and wind-dominated disks, we find that collisional fragmentation is the limiting mechanism of particle growth and the water ice sublimation line represents a critical transition point between dust deposition, drift and size regimes.

The St dichotomy along the ice line leads to different planet formation pathways between the inner and outer disk. While pebble accretion proceeds slowly for rocky embryos within the ice line (across most of the parameter space), it happens rapidly for volatile embryos beyond the ice line, allowing the growth of giant planetary cores before the disk disintegrates.

Through simulations of rocky planet growth, we evaluate the competition between pebble accretion and classical pairwise collisions between planetesimals. We conclude that the dominance of pebble accretion can only be realized in disks driven by MHD winds, slowly evolving, and lacking pressure maxima that could concentrate solids and lead to the formation of planetesimal rings. Such disks are extremely quiet, with Shakura-Sunyaev turbulence parameters of αν∼10−4.

We conclude that for most of the parameter space corresponding to the αν values ​​reflected in observations of protoplanetary disks (≳10−4), pairwise collisions represent the dominant pathway for accretion of rocky planets. Our results are discussed in the context of the origins of super-Earths and the accretion history of Earth.

Teng Ee Yap, Konstantin Batygin

Comments: Presented at the Lunar and Planetary Science Conference 2024
Fields of expertise: Earth and planetary astrophysics (Astro-Ph.EP)
Cite as: arXiv:2408.00159 (astro-ph.EP) (or arXiv:2408.00159v1 (astro-ph.EP) for this version)
https://doi.org/10.48550/arXiv.2408.00159
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Journal reference: Icarus, 417, 116085 (2024)
Related DOI:
https://doi.org/10.1016/j.icarus.2024.116085
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Submission history
By: Teng Ee Yap
(v1) Wed, 31 Jul 2024, 20:55:32 UTC (10,570 KB)
https://arxiv.org/abs/2408.00159

Astrobiology, Astronomy,