The Parsec-scale jet in quasar 3C 345

The quasar 3C 345 (z = 0.6) was monitored with very long baseline interferometry (VLBI) in 11 imaging observations at 5, 11, and 22 GHz during 1984.8-1988.2. The images probe detail of the superluminal radio jet over distances ranging from 0.12 to 12 mas (0.5-45 pc, for H_0_ = 100 km s^-1^, q_0_ = 0.5) from the unresolved radio "core." The structure of the VLBI jet is well described by two to four distinct components, but some of the images also indicate the presence of underlying jet emission, as seen in recent high dynamic range imaging. The component motions are analyzed using polynomial fits to the position offsets x(t) and y(t) relative to the core, in a form suitable for comparison with three-dimensional jet models. Components C2-C5 show superluminal motion over the full jet length. The measured speeds increase monotonically with time (or distance from the core), from ~3c to ~10c, consistent with a jet of constant Lorentz factor (γ = 10) bending away from the line of sight. Near the core the jet ridge line, as traced by the superluminal features, is strongly curved. Trajectories of successive components differ substantially within 2 mas from the core, but they become parallel at larger distances. The outer components show monotonic curvature, but the path of feature C4 shows at least two changes in curvature, or "wiggles." We reconstruct the three-dimensional path of C4 and show that its modest intrinsic bend is amplified by projection effects: the path starts within ~1^deg^ of the line of sight, and then bends smoothly away to ~4^deg^, consistent with the observed straightening of the projection of the jet on the sky. We present a model for the radio through X-ray emission from 3C 345 which self-consistently combines the inhomogeneous-jet model of Konigl for the core with homogeneous spheres for the superluminal components. This (constant Lorentz factor) model accounts for the flat radio spectrum of the nucleus and the steep spectra of the superluminal components, as derived from the VLBI images, as well as for kinematic constraints from the proper motions. The best fit requires a small opening angle for the core of ~0.5^deg^, which, like the jet bending, is amplified by projection. Inverse Compton emission from the nucleus is the likely origin of the observed soft X-ray flux, and it probably dominates over X- rays from the superluminal components. To avoid overprediction of X-rays by the model, the jet axis must lie within 5.4^deg^ of the line of sight, consistent with, but independent of, constraints from kinematics. The kinetic luminosity of the parsec-scale jet is sufficient to power the outer radio halo, but barely if the jet is light; we regard this as evidence in favor of a heavy jet; i.e., the dynamics are proton dominated.