Three-dimensional magnetohydrodynamic simulations of radiatively cooling, pulsed jets

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 c ooling, pulsed jets using the following different initial magnetic field to pologies : (1) longitudinal, (2) helical geometry permeating both the jet a nd the ambient medium, and (3) purely toroidal geometry permeating the jet only. We explore the effects of different pulsational periods, as well as d ifferent values of the magnetic field strength (beta similar or equal to 0. 1-infinity or B similar or equal to 260-0 muG). The presence of a helical o r toroidal field tends to affect the global characteristics of the fluid mo re than a longitudinal field. However, the relative differences that have b een previously detected in two-dimensional simulations involving distinct m agnetic field configurations are diminished in the three-dimensional flows. While the presence of toroidal magnetic components can modify the morpholo gy close to the jet head, inhibiting its fragmentation in the early evoluti on of the jet, as previously reported in the literature, the impact of the pulse-induced internal knots causes the appearance of a clumpy, complex mor phology 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 sur faces can also be significantly altered by the presence of magnetic fields. The increase of the magnetic field strength (decrease of beta) improves th e jet collimation and amplifies the density (by factors up to 1.4 and 4) an d the H alpha intensity (by factors up to 4 and 5) behind the knots of jets with a helical field and beta similar or equal to 1-0.1 relative to a nonm agnetic jet. As a consequence, the corresponding ratio (which is I-[S II]/I -H alpha frequently used to determine the excitation level of HH objects) c an be decreased in the MHD models with toroidal components relative to nonm agnetic calculations by about the same amounts, although the intensity esti mates above are very approximate. We also find that the helical mode of the Kelvin-Helmholtz instability can be triggered in MHD models with helical m agnetic fields, causing some wiggling of the jet axis. No evidence for the formation of the nose cones that are commonly detected in two-dimensional j et simulations with initial toroidal magnetic fields is found in the three- dimensional flows or even in the beta similar or equal to 0.1 case. The imp lications of our results for HH jets are briefly discussed.