Spiral shocks and accretion in discs

Abstract

Recent numerical and analytical results on disc-like accretion with shock waves as the only dissipation mechanism are compared. The global properties of the process are similar to those of the viscous (α) disc model, but precise values of the effective α value as a function of the accretion rate can be calculated. At low values of the ratio of specific heats (γ<1.45) accretion is possible without radiative losses. Such adiabatic accretion can occur in practice at high accretion rates on to low mass objects and may be important in the formation of planets. Following Donner, and Lynden-Bell, it is pointed out that non-axisymmetric perturbations in the outer parts of a disc increase in amplitude as they propagate in and cause spiral shocks more easily in a disc than perturbations originating in the inner parts. It is suggested for this reason that the cause of spiral structure in normal spiral galaxies lies in moderate non-axisymmetries in their gaseous outer discs.