Theory of brownian motion in Bogolyubov's method of abbreviated description. II

Bogolyubov's idea of an abbreviated description of many-particle dynamical systems is used to study the Brownian motion of a diatomic molecule in an equilibrium medium without allowance for vibrational degrees of freedom. The kinetic and hydrodynamic stages of evolution are studied. Integral equations are obtained that make it possible to calculate the nonequilibrium distribution function of the system in perturbation theory in the small ratio of the mass of a particle of the medium to the mass of the Brownian particle. A kinetic equation and the equation of rotational diffusion are constructed, a generalization of the ordinary procedure for studying the hydrodynamic stage of evolution being proposed for the derivation of the latter. The symmetry of the obtained equations is investigated. Finally, some remarks are made concerning the theory of the Brownian motion of a structureless particle in an equilibrium medium.