Δ parameter for the disc simulations with evolving Ṁ. The black lines mark the fits discussed in Appendix A. The Δ parameter describes the triggering of particle concentrations in the streaming instability, which can lead to planetesimal formation. In the regions of lower Δ at ≈5 AU, the formation of planetesimals is more likely, compared to the inner regions of the disc (≈2 AU) where Δ is higher.