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The streaming instability is a promising mechanism to induce the formation ofplanetesimals. Nonetheless, this process has been found in previous studies torequire either a dust-to-gas surface density ratio or a dust size that isenhanced compared to observed values. Employing two-dimensional globalsimulations of protoplanetary disks, we show that the vertical shearinstability and the streaming instability in concert can cause dustconcentration that is sufficient for planetesimal formation for lower surfacedensity ratios and smaller dust sizes than the streaming instability inisolation, and in particular under conditions that are consistent withobservational constraints. This is because dust overdensities forming inpressure bumps induced by the vertical shear instability act as seeds for thestreaming instability and are enhanced by it. While our two-dimensional modeldoes not include self-gravity, we find that strong dust clumping and theformation (and dissolution) of gravitationally unstable overdensities can berobustly inferred from the evolution of the maximum or the mean dust-to-gasvolume density ratio. The vertical shear instability puffs up the dust layer toan average mid-plane dust-to-gas density ratio that is significantly belowunity. We therefore find that reaching a mid-plane density ratio of one is notnecessary to trigger planetesimal formation via the streaming instability whenit acts in unison with the vertical shear instability.

astronomy and astrophysics

The coexistence of the streaming instability and the vertical shear  instability in protoplanetary disks: Planetesimal formation thresholds  explored in two-dimensional global models