Letter to the Editor

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II. How dust coagulation inside vortices can form planetesimals at low metallicity

¹ New Mexico State University, Department of Astronomy, PO Box 30001 MSC 4500 Las Cruces, NM, 88001, USA
² Department of Physics and Astronomy, Iowa State University, Ames, IA, 50010, USA
³ Institute for Advanced Computational Sciences, Stony Brook University, Stony Brook, NY, 11794-5250, USA
⁴ Department of Astrophysics, American Museum of Natural History, 200 Central Park West, New York, NY, 10024, USA

^⋆ Corresponding author: carrera4@nmsu.edu

Received: 4 April 2025
Accepted: 5 July 2025

Abstract

Context. The origin of planetesimals (∼100 km planet building blocks) has confounded astronomers for decades as numerous growth barriers appear to impede their formation. In a recent paper we proposed a novel interaction where the streaming instability (SI) and dust coagulation work in tandem: each changes the environment in a way that benefits the other. This mechanism proved effective at forming planetesimals in the fragmentation-limited inner disk, but much less effective in the drift-limited outer disk, and we concluded that dust traps may be key to forming planets at wide orbital separations.

Aims. Here we explore a different hypothesis, namely that vortices host a feedback loop in which a vortex traps dust and boosts dust coagulation, which in turn boosts vortex trapping.

Methods. We combined an analytic model of vortex trapping with an analytic model of fragmentation-limited grain growth that accounts for how dust concentration dampens gas turbulence.

Results. We find a powerful synergy between vortex trapping and dust growth. For α ≲ 10⁻³ and solar-like metallicity, this feedback loop consistently takes the grain size and dust density into the planetesimal formation region of the SI. Only in the regime of strong turbulence (α ≳ 3 × 10⁻³) does the system often converge to a steady state below the SI criterion.

Conclusions. The combination of vortex trapping with dust coagulation is an even more powerful mechanism than the one involving the SI. It is effective at lower metallicity and across the whole disk, anywhere that vortices form.