The CO snow line favours strong clumping by the streaming instability in protoplanetary discs with porous grains

Context. The radial drift and fragmentation of small dust grains in protoplanetary discs impedes their growth past centimetre sizes. Several mechanisms have been proposed to overcome these planet formation barriers, such as dust porosity or the streaming instability (SI), which is today regarded as the most promising mechanism to form planetesimals. Aims. Here, we examine whether the conditions for the SI to lead to strong clumping (the first step in planetesimal formation) are realised in protoplanetary discs containing porous grains. Methods. We used results from previous simulations of the evolution of porous grains subjected to growth, fragmentation, compaction, and bouncing in protoplanetary discs. In the ensuing disc structures, we determined the regions where the dust-to-gas ratio exceeds the critical value for strong clumping found in simulations of the SI including external turbulence. Results. We find that the conditions for strong clumping are met within the first hundred thousand years in large regions of protoplanetary discs containing porous grains, provided that the CO snow line is taken into account. If the CO snow line is neglected, the conditions are only met very close to the inner disc edge early on or over large areas well after 200 000 yr.