Compact protoplanetary discs can be produced by dead zones

Abstract: Radially compact protoplanetary discs (<=50 au) are ubiquitous in nearby star-forming regions. Multiple mechanisms have been invoked to interpret various compact discs. In this paper, we propose that fragmentation of fragile dust grains in moderate turbulence, as expected beyond the dead zone, provides an effective alternative mechanism to form compact discs which are consistent with current observations. We run 1-D dust transport and collision models with DustPy and generate synthetic observations, and find that discs formed by this mechanism have sizes determined by the extent of their dead zones. Accounting for dust porosity, and considering less fragile dust, do not change disc sizes significantly. The smooth dust morphology can be altered only when pressure bumps are present in the dead zone. However, when present at small radii (<=10 au), pressure bumps cannot effectively trap dust. Dust in these bumps fragments and replenishes the inner discs, effectively hiding dust traps in the optically thick inner disc from observations. We note a striking resemblance in the radial intensity profile between our synthetic observations and some recent high-resolution observations of compact discs. We discuss how such observations can inform our understanding of the underlying disc physics.