THE STABILITY OF RADIATIVELY COOLING JETS .2. NONLINEAR EVOLUTION

We use two-dimensional time-dependent hydrodynamical simulations to fo llow the growth of the Kelvin-Helmholtz (K-H) instability in cooling j ets into the nonlinear regime. We focus primarily on asymmetric modes that give rise to transverse displacements of the jet beam. A variety of Mach numbers and two different cooling curves are studied. The grow th rates of waves in the linear regime measured from the numerical sim ulations are in excellent agreement with the predictions of the linear stability analysis presented in the first paper in this series. In th e nonlinear regime, the simulations show that asymmetric modes of the K-H instability can affect the structure and evolution of cooling jets in a number of ways. We find that jets in which the growth rate of th e sinusoidal surface wave has a maximum at a so-called resonant freque ncy can be dominated by large-amplitude sinusoidal oscillations near t his frequency. Eventually, growth of this wave can disrupt the jet. On the other hand, nonlinear body waves tend to produce low-amplitude wi ggles in the shape of the jet but can result in strong shocks in the j et beam. In cooling jets, these shocks can produce dense knots and fil aments of cooling gas within the jet. Ripples in the surface of the je t beam caused by both surface and body waves generate oblique shock '' spurs'' driven into the ambient gas. Our simulations show these shack ''spurs'' can accelerate ambient gas at large distances from the jet b eam to low velocities, which represents a new mechanism by which low-v elocity bipolar outflows may be driven by high-velocity jets. Rapid en trainment and acceleration of ambient gas may also occur if the jet is disrupted. For parameters typical of protostellar jets, the frequency at which K-H growth is a maximum (or highest frequency to which the e ntire jet can respond dynamically) will be associated with perturbatio ns with a period of similar to 200 yr. Higher frequency (shorter perio d) perturbations excite waves associated with body modes that produce internal shocks and only small-amplitude wiggles within the jet. The f act that most observed systems show no evidence for large-amplitude si nusoidal oscillation leading to disruption is indicative that the pert urbation frequencies are generally large, consistent with the suggesti on that protostellar jets arise from the inner regions (r < 1 AU) of a ccretion disks.