Convective overstability in radially global protoplanetary disks I -- Pure gas dynamics

TL;DR

This study investigates the convective overstability in radially global protoplanetary disks through linear stability analysis and 3D simulations, revealing vortex formation, angular momentum transport, and radial structure modifications.

Contribution

It provides the first global analysis of COS near the disk mid-plane, showing its nonlinear saturation and effects on disk structure and dynamics.

Findings

COS exists only inward of Lindblad resonances

Nonlinear COS leads to vortex formation and angular momentum transport

Radial mass transport is typically outward, affecting disk evolution

Abstract

Protoplanetary disks are prone to several hydrodynamic instabilities. One candidate, Convective Overstability (COS), can drive radial semi-convection that may influence dust dynamics and planetesimal formation. However, the COS has primarily been studied in local models. This paper investigates the COS near the mid-plane of radially global disk models. We first conduct a global linear stability analysis, which shows that linear COS modes exist only radially inward of their Lindblad resonance (LR). The fastest-growing modes have LRs near the inner radial domain boundary with effective radial wavelengths that can be a substantial fraction of the disk radius. We then perform axisymmetric global simulations and find that the COS's nonlinear saturation is similar to previous incompressible shearing box simulations. In particular, we observe the onset of persistent zonal and elevator flows for sufficiently steep radial entropy gradients. In full 3D, non-axisymmetric global simulations, we find the COS produces large-scale, long-lived vortices, which induce outward radial transport of angular momentum via the excitation of spiral density waves. The corresponding $\alpha$-viscosity values of order $10^{-3}$ agree well with those found in previous 3D compressible shearing box simulations. However, in global disks, significant modifications to their radial structure are found, including the formation of pressure bumps. Interestingly, the COS typically generates an outward radial mass transport, i.e. decretion. We briefly discuss the possible implications of our results for planetesimal formation and for interpreting dust rings and asymmetries observed in protoplanetary disks.