Resistive double-diffusive instability in the dead-zones of protostellar disks

TL;DR

This paper introduces a new linear instability in the dead-zones of protostellar disks driven by a negative entropy gradient, weak magnetic fields, and resistive diffusion, which may impact disk mixing and planet formation.

Contribution

It identifies a novel axisymmetric, double-diffusive buoyancy instability that operates under specific conditions in protostellar disk dead-zones, expanding understanding of disk dynamics.

Findings

The instability requires a negative radial entropy gradient, weak magnetic fields, and efficient resistive diffusion.

It operates at small scales much shorter than the resistive scale.

Nonlinear saturation may lead to mixing affecting disk temperature and planet formation.

Abstract

We outline a novel linear instability that may arise in the dead-zones of protostellar disks, and possibly the fluid interiors of planets and protoplanets. In essence it is an axisymmetric buoyancy instability, but one that would not be present in a purely hydrodynamical gas. The necessary ingredients for growth include a negative radial entropy gradient (of any magnitude), weak magnetic fields, and efficient resistive diffusion (in comparison with thermal diffusion). The character of the instability is local, axisymmetric, and double-diffusive, and it attacks lengths much shorter than the resistive scale. Like the axisymmetric convective instability, it draws its energy from the negative radial entropy gradient; but by utilising the diffusing magnetic field, it can negate the stabilising influence of rotation. Its nonlinear saturated state, while not transporting appreciable angular momentum, could drive radial and vertical mixing, which may influence the temperature structure of the disk, dust dynamics and, potentially, planet formation.