Primordial dust rings, hidden dust mass, and the first generation of planetesimals in gravitationally unstable protoplanetary disks

TL;DR

This paper proposes a new mechanism for dust accumulation and planetesimal formation in gravitationally unstable protoplanetary disks, highlighting the role of GI-induced dust rings and streaming instability in the inner disk regions.

Contribution

It introduces a novel process where gravitational instability creates dust rings that can develop streaming instability, potentially leading to planetesimal formation in the inner disk.

Findings

Formation of low transport regions similar to dead zones in unstable disks.

Accumulation of massive dust rings susceptible to streaming instability.

Potential for early planetesimal formation in the inner terrestrial zone.

Abstract

Aims. A new mechanism of dust accumulation and planetesimal formation in a gravitationally unstable disk with suppressed magnetorotational instability is studied and compared with the classical dead zone in a layered disk model. Methods. We use numerical hydrodynamics simulations in the thin-disk limit FEOSAD code to model the formation and long-term evolution of gravitationally unstable disks, including dust dynamics and growth. Results. We found that in gravitationally unstable disks with a radially varying strength of gravitational instability a region of low mass and angular momentum transport forms in the inner several astronomical units. This region is characterized by low effective \alpha_GI and is similar in characteristics to the dead zone in the layered disk model. As the disk forms and evolves, the GI-induced dead zone accumulates a massive dust ring, which is susceptible to the development of the streaming instability. The model and observationally inferred dust masses and radii may differ significantly in gravitationally unstable disks with massive inner dust rings. Conclusions. The early occurrence of the GI-induced dust ring followed by the presumed development of the streaming instability suggest that this mechanism may form the first generation of planetesimals in the inner terrestrial zone of the disk. The proposed mechanism, however, crucially depends on the susceptibility of the disk to gravitational instability and requires that the magnetorotational instability be suppressed.