A helically twisted relativistic jet: nonradial motions and accelerations in 3C345

The radio source 3C 345 has a curved, inner-jet structure consisting of three components. Components C2 and C3 appear to be moving with βapp ~ 17 and 13, respectively, along approximately parallel straight-line trajectories with motion along a mean direction ~ −75°. Component C4 recently appeared at a position angle ~ −135° and has exhibited nonlinear motion and apparent acceleration from βapp ~ 4–10 over a period of about 2 years with motion along a mean direction < −75°. We show how the motions of C4 can be explained by a feature that is propagating with constant speed along a helically twisted jet in which the wavelength of the helix increases in proportion to the jet's radius. This form for a helical twist is predicted by the linearized fluid-dynamical equations, which can be used to describe the response of a light jet to helical distortion.