2002ApJ...568..639P

We present a model to determine the physical parameters of jets and hot spots of a sample of compact symmetric objects (CSOs) under very basic assumptions like synchrotron emission and minimum energy conditions. Based on this model, we propose a simple evolutionary scenario for these sources assuming that they evolve in ram pressure equilibrium with the external medium and constant jet power. The parameters of our model are constrained from fits of observational data (radio luminosity, hot spot radius, and hot spot advance speed) versus projected linear size. From these plots we conclude that CSOs evolve self-similarly and that their radio luminosity increases with linear size along the first kiloparsec. Assuming that the jets feeding CSOs are relativistic from both kinematical and thermodynamical points of view, we use the values of the pressure and particle number density within the hot spots to estimate the fluxes of momentum (thrust), energy, and particles of these relativistic jets. The mean jet power obtained in this way is within an order of magnitude of that inferred for Fanaroff-Riley type 2 sources, which is consistent with CSOs being the possible precursors of large doubles. The inferred flux of particles corresponds to, for a barionic jet, about 10% of the mass accreted by a black hole of 10⁸ M_☉ at the Eddington limit, pointing toward a very efficient conversion of accretion flow into ejection or to a leptonic composition of jets. We have considered three different models (namely, models I, IIa, and IIb). Model I, assuming constant hot spot advance speed and increasing luminosity, can be ruled out on the grounds of its energy cost. However, models IIa and IIb seem to describe limiting behaviors of sources evolving at constant advance speed and decreasing luminosity (model IIa) and decreasing hot spot advance speed and increasing luminosity (model IIb). In all our models the slopes of the hot spot luminosity and advance speed with source linear size are governed by only one parameter, namely, the external density gradient. A short discussion on the validity of models IIa and IIb to describe the complete evolution of powerful radio sources from their CSO phase is also included.