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

We present a novel study of dust-vortex evolution in global two-fluid disk simulations to find out if evolution toward high dust-to-gas ratios can occur 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 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 {{global active dust ring}} is a generic outcome confirming our previous results obtained for larger grains. We propose a scenario in which, irrespective of grain size, multiple pathways can lead to local dust-to-gas ratios of order unity and above on relatively short timescales, $< 10^5$ yr, in the 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 dust ring. When $St<10^{-2}$, the vortex is resistant to destabilization. As a result, dust concentrations occur locally due to turbulence developing inside the vortex. Whatever the Stokes number, 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. Although quantitative connections with other instabilities still need to be made, we argue that our results support a new scenario of vortex-driven planetesimal formation.