Dust-vortex instability in the regime of well-coupled grains

Clément Surville, Lucio Mayer

Published 2018-01-23Version 1

We present a novel study of dust-vortex evolution in global two-fluid disk simulations to elucidate if strong evolution of dust-to-gas ratios can occur even in a regime of well-coupled grains with low Stokes numbers ($St=10^{-3}-{4\times 10^{-2}}$). We design a new implicit scheme in the code RoSSBi, to overcome the short timesteps occurring for small grain sizes. We also discover that the linear capture phase occurs self-similarly for all grain sizes, with an intrinsic timescale (characterizing the vortex lifetime) scaling as $1/St$. After vortex dissipation, the formation of a disk-wide turbulent dust ring is a generic outcome confirming our previous results obtained for larger grains. We propose a novel scenario in which, irrespective of grain size, multiple pathways can lead to local dust-to-gas ratios order unity and above on relatively short timescales, $< 10^5$ yr, in presence of a vortex, even with $St=10^{-3}$. When $St>10^{-2}$, the vortex is quickly dissipated by two-fluid instabilities, and large dust density enhancements form in the global disk-wide dust ring. When $St<10^{-2}$, the vortex is resilient to destabilization and dust concentrations occur locally, as a result of turbulence developing inside the vortex. In both cases, dust-to-gas ratios in the range $1-10$, a necessary condition to trigger a subsequent streaming instability, or even a direct gravitational instability of the dust clumps, appears to be an inevitable outcome. While the quantitative connection with such other instabilities will have to be studied, we argue that our results strongly support a new scenario of vortex-driven planetesimal formation.