Magnetohydrodynamic models of axisymmetric protostellar jets

We present the results of a series of axisymmetric time-dependent magnetohydrodynamic (MHD) simulations of the propagation of cooling, overdense jets. Our numerical models are motivated by the properties of outflows associated with young stellar objects. A variety of initial field strengths and configurations are explored for both steady and time-variable (pulsed) jets. For the parameters of protostellar jets adopted here, even apparently weak magnetic gelds with strengths B greater than or similar to 60 mu G in the preshocked jet beam can have a significant effect on the dynamics, for example, by altering the density, width, and fragmentation of thin shells formed by cooling gas. Strong toroidal fields (greater than or equal to 100 mu G) with a radial profile that peaks near the surface of the jet result in the accumulation of dense shocked gas in a "nose cone" at the head of jet. We suggest that this structure is unstable in three dimensions. A linear analysis predicts that axisymmetric pinch modes of the MHD Kelvin-Helmholtz instability should grow only slowly for the highly supermagnetosonic jets studied here; we find no evidence for them in our simulations. Some of our models appear unstable to current-driven pinch modes; however, the resulting pressure and density variations induced in the jet beam are not large, making this mechanism an unlikely source of emission knots in the jet beam. In the case of pulsed jets, radial hoop stresses confine shocked jet material in the pulses to the axis, resulting in a higher density in the pulses in comparison to purely hydrodynamic models. In addition, if the magnetic field strength varies with radius, significant radial structure is produced in the pulses (the density is strongly axially peaked, for example) even if the density and velocity in the jet follow a constant "top-hat" profile initially.