On the axisymmetric stability of stratified and magnetized accretion disks

Abstract

We conduct a comprehensive axisymmetric, local linear mode analysis of a stratified, differentially rotating disk permeated by a toroidal magnetic field which could provide significant pressure support. In the adiabatic limit, we derive a new stability criteria that differs from the one obtained for weak magnetic fields with a poloidal component and reduces continuously to the hydrodynamic Solberg-Høiland criteria. Three fundamental unstable modes are found in the locally isothermal limit. They comprise of overstable: (i) acoustic oscillations, (ii) radial epicyclic (acoustic-inertial) oscillations and (iii) vertical epicyclic (or vertical shear) oscillations. All three modes are present for finite ranges of cooling times but they are each quickly quenched past respective cut-off times. The acoustic and acoustic-inertial overstable modes are driven by the background temperature gradient. When vertical structure is excluded, we find that the radial epicyclic modes appear as a nearly degenerate pair. One of these is the aforementioned acoustic-inertial mode and the other has been previously identified in a slightly different guise as the convective overstability. Inclusion of vertical structure leads to the development of overstable oscillations destabilized by vertical shear but also has the effect of suppressing the radial epicyclic modes. Although our study does not explicitly account for non-ideal effects, we argue that it may still shed light into the dynamics of protoplanetary disk regions where a strong toroidal field generates as a result of the Hall-shear instability.