A and a Manuscript-nr. a Helical Model for the Compact Jet in 3c 345

We present a simple model for the apparent superluminal motion along curved trajectories of features observed in the compact radio jet of the quasar 3C 345. The model is inspired by the magneto-hydrodynamic approach of Camenzind (1986), and assumes a conical geometry of the jet within about three milliarcseconds of the radio core. Simultaneous conservation of three out of four physical parameters is used to derive analytic solutions for the equation of motion of the idealized jet features. These are the jet's kinetic energy, angular momentum , and momentum along the jet axis. Conservation of angular momentum is required to t the observed trajectories at distances larger than 3 mas, in agreement with the kinematic predictions of Camenzind's model. The best-t model variant preserves angular momentum, kinetic energy, and opening angle , with an inclination of the jet axis to the line of sight = 6:8 , an opening angle = 0:95 , and Lorentz factors = 5:8 and = 4:6 for components C4 and C5, respectively (H = 100 km s 1 Mpc 1). The model ignores turbulent small-scale motion of the jet components, and it does not take into account the overall bending of the jet beyond about 4 mas, restricting it's applicability to the inner few milliarcseconds from the core.