Effects of Cooling on the Propagation of Magnetized Jets

We present multidimensional simulations of magnetized radiative jets appropriate to young stellar objects (YSOs). Magnetized jets subject to collisionally excited radiative losses have not, as yet, received extensive scrutiny. The purpose of this Letter is to articulate the propagation dynamics of radiative MHD jets in the context of the extensive jet literature. Most importantly, we look for morphological and kinematic diagnostics that may distinguish hydrodynamic protostellar jets from their magnetically dominated cousins. Our simulations are axisymmetric (2.5 dimensions). A toroidal (Bϕ) field geometry is used. Our models have high sonic Mach numbers (Ms ≈ 10) but lower fast-mode Mach number (Mf ≈ 5). This is approximately the case for jets formed via disk-wind or X-wind models—currently the consensus choice for launching and collimating YSO jets. Time-dependent radiative losses are included via a coronal cooling curve. Our results demonstrate that the morphology and propagation characteristics of strongly magnetized radiative jets can differ significantly from jets with weak fields. In particular, the formation of nose cones via postshock hoop stresses leads to narrow bow shocks and enhanced bow shock speeds. In addition, the hoop stresses produce strong shocks in the jet beam, which contrasts with the relatively unperturbed beam in radiative hydrodynamic jets. Our simulations show that pinch modes produced by magnetic tension can strongly affect magnetized protostellar jets. These differences may be useful in observational studies designed to distinguish between competing jet collimation scenarios.