Vortices in self-gravitating gaseous discs

Abstract

(abridged) Vortices are believed to play a role in the formation of km-sized planetesimals. However, vortex dynamics is commonly studied in non-self-gravitating discs. The main goal here is to examine the effects of disc self-gravity on vortex dynamics. For this purpose, we employ the 2D self-gravitating shearing sheet approximation. A simple cooling law with a constant cooling time is adopted, such that the disc settles down into a quasi-steady gravitoturbulent state. In this state, vortices appear as transient structures undergoing recurring phases of formation, growth to sizes comparable to a local Jeans scale and eventual shearing and destruction due to the combined effects of self-gravity and background Keplerian shear. Each phase typically lasts about 2 orbital periods or less. As a result, in self-gravitating discs the overall dynamical picture of vortex evolution is irregular consisting of many transient vortices at different evolutionary stages and, therefore, with various sizes up to the local Jeans scale. Vortices generate density waves during evolution, which turn into shocks. Therefore, the dynamics of density waves and vortices are coupled implying that, in general, one should consider both vortex and spiral density wave modes in order to get a proper understanding of self-gravitating disc dynamics. Our results suggest that given such an irregular and rapidly varying character of vortex evolution in self-gravitating discs, it may be difficult for such vortices to effectively trap dust particles. Further study of the behaviour of dust particles embedded in a self-gravitating gaseous disc is, however, required to strengthen this conclusion.