A HELICAL MODEL FOR THE COMPACT JET IN 3C-345

We present a simple model for the apparent superluminal motion along c urved trajectories of features observed in the compact radiojet of the quasar 3C 345. The model is inspired by the magneto-hydrodynamic appr oach of Camenzind (1986), and assumes a conical geometry of the jet wi thin about three milliarcseconds of the radio core. Simultaneous conse rvation of three out of four physical parameters is used to derive ana lytic solutions for the equation of motion of the idealized jet featur es. These are the jet's kinetic energy, angular momentum, and momentum along the jet axis. Conservation of angular momentum is required to f it the observed trajectories at distances larger than 3 mas, in agreem ent with the kinematic predictions of Camenzind's model. The best-fit model variant preserves angular momentum, kinetic energy, and opening angle, with an inclination of the jet axis to the line of sight theta = 6.8 degrees, an opening angle psi = 0.95 degrees, and Lorentz factor s gamma = 5.8 and gamma = 4.6 for components C4 and C5, respectively ( H-o = 100 km s(-1) Mpc(-1)). The model ignores turbulent small-scale m otion of the jet components, and it does not take into account the ove rall bending of the jet beyond about 4 mas, restricting it's applicabi lity to the inner few milliarcseconds from the core.