Abstract

Fluctuations in a stellar system’s gravitational field cause the orbits of stars to evolve. The resulting evolution of the system can be computed with the orbit-averaged dressed Fokker-Planck equation in angle-actions coordinates. I will present the formalism that enables one to compute the diffusion tensor from a given source of noise in the gravitational field when the system’s dynamical response to that noise is included. In the case of a cool stellar disc we are able to reduce the computation of the diffusion tensor to a one-dimensional integral. The shot-noise-driven formation of narrow ridges of resonant orbits is therefore recovered in the WKB limit of tightly wound transient spirals, for a tepid Toomre-stable tapered disc, so that we are able to explain analytically the principal features of numerical simulations of such discs.