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Context.One of the most important open questions in planet formation is how dust grains in a protoplanetary disk manage to overcome growth barriers and form the ∼100 km planet building blocks that we call planetesimals. There appears to be a gap between the largest grains that can be produce by coagulation, and the smallest grains that are needed for the streaming instability (SI) to form planetesimals. Aims.Here we explore the novel hypothesis that dust coagulation and the SI work in tandem; in other words, they form a feedback loop where each one boosts the action of the other to bridge the gap between dust grains and planetesimals. Methods.We developed a semi-analytical model of dust concentration due to the SI, and an analytic model of how the SI affects the fragmentation and radial drift barriers. We then combined them to model our proposed feedback loop. Results.In the fragmentation-limited regime, we find a powerful synergy between the SI and dust growth that drastically increases both grain sizes and densities. We find that a midplane dust-to-gas ratio ofϵ ≥ 0.3 is a sufficient condition for the feedback loop to reach the planetesimal-forming region for turbulence values 10−4 ≤ α ≤ 10−3and grain sizes 0.01 ≤ St ≤ 0.1. In contrast, the drift-limited regime only shows grain growth without significant dust accumulation. In other words, planetesimal formation remains challenging in the drift-dominated regime and dust traps may be required to allow planet formation at wide orbital distances.
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Self-consistent model for dust-gas coupling in protoplanetary disks
Various physical processes that ensue within protoplanetary disks - including vertical settling of icy and rocky grains, radial drift of solids, planetesimal formation, as well as planetary accretion itself - are facilitated by hydrodynamic interactions between H/He gas and high-Z dust. The Stokes number, which quantifies the strength of dust-gas coupling, thus plays a central role in protoplanetary disk evolution and its poor determination constitutes an important source of uncertainty within the theory of planet formation. In this work, we present a simple model for dust-gas coupling and we demonstrate that for a specified combination of the nebular accretion rate,Ṁ, and turbulence parameter a, the radial profile of the Stokes number can be calculated in a unique way. Our model indicates that the Stokes number grows sublinearly with the orbital radius, but increases dramatically across the water-ice line. For fiducial protoplanetary disk parameters ofṀ= 10−8M⊙per year andα= 10−3, our theory yields characteristic values of the Stokes number on the order of St ~ 10−4(corresponding to ~mm-sized silicate dust) in the inner nebula and St ~ 10−1(corresponding to icy grains of a few cm in size) in the outer regions of the disk. Accordingly, solids are expected to settle into a thin subdisk at large stellocentric distances, while remaining vertically well mixed inside the ice line.
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- Award ID(s):
- 2109276
- PAR ID:
- 10529228
- Publisher / Repository:
- ESO
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 666
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A19
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1051/0004-6361/202243196
