MORPHOLOGY AND DYNAMICS OF RELATIVISTIC JETS

We present a comprehensive analysis of the morphology and dynamics of relativistic pressure-matched axisymmetric jets. The numerical simulat ions have been carried out with a high-resolution shock-capturing hydr ocode based on an approximate relativistic Riemann solver derived from the spectral decomposition of the Jacobian matrices of relativistic h ydrodynamics. We discuss the dependence of the jet morphology on sever al parameters, paying special attention to the relativistic effects ca used by high Lorentz factors and large internal energies of the beam f low. The parameter space of our analysis is spanned by the ratio of th e beam and ambient medium rest mass density (eta), the beam Mach numbe r (M(b)), the beam Lorentz factor (W-b), and the adiabatic index (gamm a) of the equation of state (assuming an ideal gas). Both the ultrarel ativistic regime (W-b greater than or equal to 20) and the hypersonic regime (relativistic Mach number greater than 100) have been studied. Our results show that the enhancement of the effective inertial mass o f the beam due to relativistic effects (through the specific enthalpy and the Lorentz factor) makes relativistic jets significantly more sta ble than Newtonian jets. We find that relativistic jets propagate very efficiently through the ambient medium, at speeds that agree very wel l with those obtained from an estimate based on a one-dimensional mome ntum balance. The propagation efficiency of a relativistic jet is an i ncreasing function of the beam flow velocity. Relativistic jets seem t o give rise to two different morphologies, according to the relevance of relativistic effects. Hot beams (i.e., with internal energies compa rable to the beam rest-mass energy) show little internal structure (as they are almost in pressure equilibrium with their surroundings) and relatively smooth cocoons forming lobes near the head of the jet. High ly supersonic models, in which the kinematic relativistic effects due to high beam flow Lorentz factors dominate, display extended cocoons t hat are overpressured with respect to the environment. The cocoon thic kness decreases, and its mean pressure increases with increasing beam Lorentz factor.