Spectral evolution of the parsec-scale jet in the quasar 3C 345

The long-term evolution of the synchrotron emission from the parsec-scale jet in the quasar 3C 345 is analyzed on the basis of multifrequency monitoring with very long baseline interferometry (VLBI) and covering the period 1979-1994. We demonstrate that the compact radio structure of 3C 345 can be adequately represented by Gaussian model fits and that the model fits at different frequencies are sufficiently reliable for studying the spectral properties of the jet. We combine the model fits from 44 VLBI observations of 3C 345 made at eight different frequencies between 2.3 and 100 GHz. This combined database is used for deriving the basic properties of the synchrotron spectra of the VLBI core and the moving features observed in the jet. We calculate the turnover frequency, the turnover flux density, and the integrated 4-25 GHz flux and 4-25 GHz luminosity of the core and the moving features. The core has an estimated mean luminosity L-core = (7.1 +/- 3.5) x 10(42) h(-2) ergs s(-1); the estimated total luminosity of 3C 345 on parsec scales is approximate to 3 x 10(43) h(-2) ergs s(-1) (about 1% of the observed luminosity of the source between the radio to infrared regimes). The luminosities of the core and most of the moving features decrease at the average rate of 1.2 x 10(35) h(-2) (0.74 +/- 0.06)(t - 1979.0) ergs s(-2) (t measured in years). The derived luminosity variations require intrinsic acceleration of the moving features. The turnover frequency of one of the moving features reaches a peak during the above period. The combination of the overall spectral and kinematic changes in that feature cannot be reproduced satisfactorily by relativistic shocks, which may indicate rapid dissipation in shocks. The spectral changes in the core can be reconciled with a shock or dense plasma condensation traveling through the region where the jet becomes optically thin. We are able to describe the evolution of the core spectrum by a sequence of five flarelike events characterized by an exponential rise and decay of the particle number density of the material injected into the jet. The same model is also capable of predicting the changes in the flux density observed in the core. The flares occur approximately every 3.5-4 yr, roughly correlating with appearances of new moving features in the jet and indicating that a quasi-periodic process in the nucleus may be driving the observed emission and structural evolution of 3C 345.