Figure 3. The green region shows the solid/gas mass ratio Z required for particles of Stokes’ Number τs to generate the streaming instability (from Yang, Johansen, & Carrera, 2016). If Z is not large enough, the instability cannot occur (red region). The blue dashed line shows the limit for streaming instability found in an earlier work using simulations with more limited resolution. There is some hope that future simulations, with even larger resolution, might show that the actual Z-threshold for the streaming instability is below 1%, the solar solid/gas ratio.