Introduction

In previous chapters we have discussed extensively the theory and applications of thin (H ≪ R) accretion discs. We hope the reader will by now be convinced that this theory is reasonably well understood, and that it rests on a fairly firm observational basis. The case for thick (H ∼ R) accretion discs however is less compelling as the theory is still under development and the relevant observations are few, difficult and indirect. Furthermore, since the publication of the first papers on equilibrium thick discs or tori much work has been done on extending these solutions allowing some form of poloidal flow and studying extensively their dynamical stability. The wealth of these investigations is a testimony to the interest generated by these structures. The results obtained so far virtually rule out the reality of thick discs as non-accreting toroidal equilibria but leave open the more exciting possibility of the existence of closely related accreting flows which could be of astrophysical interest. We begin here by discussing the toroidal equilibria without accretion and summarize the stability results at the end of this chapter. More recent and more general solutions with radial and other poloidal flows are discussed in Chapter 11.

The current interest in the theory of the structure, evolution and stability of thick accretion discs is due to the possibility that thick discs may be relevant to the understanding of the central power sources in radio galaxies and quasars (see Chapters 7–9).