Dust-vortex Instability in the Regime of Well-coupled Grains

Abstract

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 × 10 −2 ). We design a new implicit scheme in the code RoSSBi, to overcome the short time-steps 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, regardless of grain size, multiple pathways can lead to local dust-to-gas ratios of about 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. Regardless of 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.