The generalized Poynting-Robertson effect

The special relativistic equations of motion for a particle and the general relativistic equations of motion for a fluid in an arbitrary radiation field are formulated. The Poynting-Robertson forces are manifested in some of the velocity-dependent terms obtained. Newtonian approximations to the equations of motion are solved for a particle in orbit about a spherically symmetric source of radiation, where the angular size of the source as seen from the orbit is arbitrary. Situations are considered in which the absorption cross section of the particle is independent of frequency, as well as when the particle is an atom absorbing in a spectral line. In both instances, it is found that the finite size of the source of radiation leads to greater Poynting-Robertson drags than a point source of the same luminosity. The Newtonian equations are solved for a particle moving radially outward from the source. Limits for the velocities of high-speed particles resulting from Poynting-Robertson drags are obtained. The fluid equations are discussed briefly in their relation to theoretical treatments of relativistic jets and accretion disks. 9 refs