Positive Feedback II: How Dust Coagulation inside Vortices Can Form Planetesimals at Low Metallicity

Abstract: The origin of planetesimals ($\sim$100 km planet building blocks) has confounded astronomers for decades, as numerous growth barriers appear to impede their formation. In a paper we proposed a novel interaction where the streaming instability (SI) and dust coagulation work in tandem, with each one changing 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, concluding that dust traps may be key to forming planets at wide orbital separations. Here we explore a different hypothesis: That vortices host a feedback loop in which a vortex traps dust, boosting dust coagulation, which in turn boosts vortex trapping. We combine an analytic model of vortex trapping with an analytic model of fragmentation limited grain growth that accounts for how dust concentration dampens gas turbulence. We find a powerful synergy between vortex trapping and dust growth. For $\alpha \le 10^{-3}$ and solar-like metallicity this feedback loop consistently takes the grain size and dust density into the planetesimal formation region of the streaming instability (SI). Only in the regime of strong turbulence ($\alpha \ge 3\times 10^{-3}$) does the system often converge to a steady state below the SI criterion. 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.