Compact protoplanetary discs can be produced by dead zones

TL;DR

This study proposes that dust fragmentation beyond dead zones in protoplanetary discs can naturally produce compact discs, aligning with observations, and explores how pressure bumps influence dust morphology and observability.

Contribution

It introduces a novel mechanism for compact disc formation via dust fragmentation in dead zones, supported by models and synthetic observations, expanding understanding of disc evolution.

Findings

Disc sizes are determined by dead zone extents.

Dust porosity and fragility do not significantly alter disc sizes.

Pressure bumps can modify dust morphology but are ineffective at trapping dust at small radii.

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.