Stability and collimation of Herbig-Haro jets

We here investigate the stability and collimation of Herbig-Haro jets through 3-D simulations using a Smoothed Particle Hydrodynamics code (e.g. \cite{GDPB93}, 1994; \cite{cher94}, \cite{GBB96}, \cite{GBir96}), which has been modified to incorporate the effects of magnetic fields (\cite{GC96}). Two initial magnetic field geometries are considered: (i) a uniform longitudinal B-field in the jet and environment, and (ii) a helical B-field. Compared to pure hydrodynamical cases, the presence of magnetic fields increases the beam collimation and, in general, reduces the growth of density at the bow shock in the head. Both geometries promote some pinching along the beam which may offer an alternative mechanism for internal knot formation in few dynamical times. We find that the radiative cooling inhibits the effects caused by the presence of magnetic fields. In particular, the wiggling that had appeared in a previous 3-D MHD numerical work (\cite{To93}) is ``drastically" reduced when we consider the non-adiabaticity of the post-shocked material. On the other hand, the presence of a purely longitudinal field in equipartition with the gas is able to stabilize the cold condensations that form at the head of a cooling jet, when the post-shocked material that is deposited behind the Mach disk breaks into clumps due to the development of dynamical and thermal instabilities. The observational counterpart of these clumps could be the observed Herbig-Haro (HH) objects at the head of the HH jets.