Numerical simulations of protostellar jets with nonequilibrium cooling. II. Models of pulsed jets

In this second of three papers, we present multidimensional time-dependent numerical simulations of the propagation of protostellar jets into a uniform ambient medium which utilize a nonequilibrium treatment of optically thin radiative cooling. This paper focuses on two- and three-dimensional models of pulsed jets in which the jet inlet velocity is assumed to be intrinsically variable. These models are motivated by recent observations which suggest temporal variability may account for knots of emission detected in the jet beam in several sources. Our simulations show that large-amplitude periodic variations as required by observations produce pulses which quickly steepen into shocks. For each pulse two shocks are formed: an upstream shock (propagating more slowly than the jet velocity) which decelerates high-velocity material as it collides with the pulse, and a downstream shock (propagating more quickly than the jet velocity) produced as the pulse sweeps up low-velocity material ahead of it. We find the pressure in the postshock material located between these two shocks is substantial and it not only causes the shocks to separate, but also ejects jet material laterally from the pulses. This combination causes the pulses to widen and decay in amplitude as they propagate. The effect of varying the pulse amplitude and frequency on the evolution is studied. In three dimensions, emission from the shock surfaces bounding the pulses produce well-separated emission knots which move at the mean velocity of the jet and fade as the pulses decay. Dense, cooled gas which collects at the head of the jet undergoes non-axisymmetric fragmentation by a variety of dynamical instabilities, leading to clumps and filaments of material. Synthetic position-velocity diagrams constructed from the three-dimensional kinematics of the pulsed jet reveal emission knots and a distinctive "sawtooth" structure.