Three‐dimensional Magnetohydrodynamic Simulations of Radiatively Cooling, Pulsed Jets

Abstract

We here investigate, by means of fully three-dimensional smoothed particle magnetohydrodynamic numerical simulations, the effects of different initial magnetic field configurations on the evolution of overdense, radiatively cooling, pulsed jets using the following different initial magnetic field topologies: (1) longitudinal, (2) helical geometry permeating both the jet and the ambient medium, and (3) purely toroidal geometry permeating the jet only. We explore the effects of different pulsational periods, as well as different values of the magnetic field strength (? 0.1-? or B 260-0 ?G). The presence of a helical or toroidal field tends to affect the global characteristics of the fluid more than a longitudinal field. However, the relative differences that have been previously detected in two-dimensional simulations involving distinct magnetic field configurations are diminished in the three-dimensional flows. While the presence of toroidal magnetic components can modify the morphology close to the jet head, inhibiting its fragmentation in the early evolution of the jet, as previously reported in the literature, the impact of the pulse-induced internal knots causes the appearance of a clumpy, complex morphology at the jet head (as required by the observations of Herbig-Haro [HH] jets) even in the MHD jet models with helical or toroidal configurations. The detailed structure and emission properties of the internal working surfaces can also be significantly altered by the presence of magnetic fields. The increase of the magnetic field strength (decrease of ?) improves the jet collimation and amplifies the density (by factors up to 1.4 and 4) and the H? intensity (by factors up to 4 and 5) behind the knots of jets with a helical field and ? 1-0.1 relative to a nonmagnetic jet. As a consequence, the corresponding I[S II]/IH? ratio (which is frequently used to determine the excitation level of HH objects) can be decreased in the MHD models with toroidal components relative to nonmagnetic calculations by about the same amounts, although the intensity estimates above are very approximate. We also find that the helical mode of the Kelvin-Helmholtz instability can be triggered in MHD models with helical magnetic fields, causing some wiggling of the jet axis. No evidence for the formation of the nose cones that are commonly detected in two-dimensional jet simulations with initial toroidal magnetic fields is found in the three-dimensional flows or even in the ? 0.1 case. The implications of our results for HH jets are briefly discussed.