Contour plot of the ratio between the dust-to-gas ratio and the critical dust-to-gas ratio needed for planetesimal formation via the streaming instability in the reference case. The dust-to-gas ratio starts to exceed the critical value at ≈ 100 au at 0.8 Myr. Over the next ≈0.7 Myr this super-critical region expands inwards to an inner edge of ≈8 au. The region disappears as the dust is drained from the disc.