Dust-driven vortex cascades originating at water snow regions: A pathway to planetesimal formation

Abstract

The origin of observed planetary systems, including our Solar System, as well as their diversity, is still an open question. Streaming instability (SI) is an important mechanism for the formation of gravitationally bound planetesimals, which can grow to form planetary embryos and eventually planets. Snow lines in a protoplanetary disk can assist this process, as they can form pressure maxima and promote both dust accumulation and growth. Since the sublimation of a volatile is gradual due to opacity changes, a snow line in a protoplanetary disk is in fact a radially extended "snow region" of constant temperature. It has been shown that dust can influence disk viscosity through the adsorption of charged particles, and even a minor perturbation in the gas can trigger the excitation of multiple small-scale Rossby vortices. Here, we investigate the possibility of Rossby vortex excitation and rapid planetesimal formation at temperature substructures associated with the snow regions, using global 2D gas-dust coupled hydrodynamic simulations that include dust feedback and self-gravity. We find that an initial temperature substructure in a protoplanetary disk can seed a rapid cascade of long-lived, self-sustaining Rossby vortices. The vortices accumulate significant amount of dust and the local conditions are favorable for SI as well as gravitational collapse. However, the vortex formation via this mechanism requires sufficient decoupling between dust and gas, and such conditions may not be met early on when the disk is gas-rich, resulting in a delayed onset of vortex formation. The self-sustaining Rossby vortices offer exceptionally favorable conditions for dust growth and the formation of planetesimals, as well as a possible pathway for the rapid formation of planetary cores.