Axisymmetric modes in vertically stratified self-gravitating discs

Abstract

We perform linear analysis of axisymmetric vertical normal modes in stratified compressible self-gravitating polytropic discs in the shearing box approximation. We study specific dynamics for subadiabatic, adiabatic and superadiabatic vertical stratifications. In the absence of self-gravity, four well-known principal modes can be identified in a stratified disc: acoustic p-, surface gravity f-, buoyancy g- and inertial r-modes. After characterizing modes in the non-self-gravitating case, we include self-gravity and investigate how it modifies the properties of these modes. We find that self-gravity, to a certain degree, reduces their frequencies and changes the structure of the dispersion curves and eigenfunctions at radial wavelengths comparable to the disc height. Its influence on the basic branch of the r-mode, in the case of subadiabatic and adiabatic stratifications, and on the basic branch of the g-mode, in the case of superadiabatic stratification (which in addition exhibits convective instability), does appear to be strongest. Reducing the three-dimensional Toomre's parameter Q_{3D} results in the latter modes becoming unstable due to self-gravity, so that they determine the onset criterion and nature of gravitational instability of a stratified disc. By contrast, the p-, f- and convectively stable g-modes, although their corresponding \omega^2 are reduced by self-gravity, never become unstable however small the value of Q_{3D}. This is a consequence of the three-dimensionality of the disc. The eigenfunctions corresponding to the gravitationally unstable modes are intrinsically 3D. We also contrast the more exact instability criterion based on our 3D model with that of density waves in 2D (razor-thin) discs. Based on these findings, we comment on the origin of surface distortions seen in numerical simulations of self-gravitating discs.