Magnetic field effects on the head structure of protostellar jets

We present the results of three-dimensional smooth particle magnetohydrodynamics numerical simulations of supermagnetosonic, overdense, radiatively cooling jets. Together with a baseline nonmagnetic calculation, two initial magnetic configurations (in approximate equipartition with the gas) are considered: (1) a helical field and (2) a longitudinal field, both of which permeate both the jet and the ambient medium. We find that magnetic fields have important effects on the dynamics and structure of radiative cooling jets, especially at the head. The presence of a helical field suppresses the formation of the clumpy structure that is found to develop at the head of purely hydrodynamical jets by fragmentation of the cold shell of shocked material. On the other hand, a cooling jet embedded in a longitudinal magnetic field retains clumpy morphology at its head. This fragmented structure resembles the knotty pattern commonly observed in HH objects behind the bow shocks of protostellar jets. This suggests that a strong (equipartition) helical magnetic field configuration is ruled out at the jet head. Therefore, if strong magnetic fields are present, they are probably predominantly longitudinal in those regions. In both magnetic configurations, we find that the confining pressure of the cocoon is able to excite short-wavelength MHD Kelvin-Helmholtz pinch modes that drive low-amplitude internal shocks along the beam. These shocks are not strong however, and it is likely that they could only play a secondary role in the formation of the bright knots observed in protostellar jets.