Numerical simulations of steady and pulsed non-adiabatic magnetised jets from young stars

In contrast to jets from radio galaxies, energy losses due to radiation effects, atomic hydrogen ionisation/recombination and molecular hydrogen dissociation are important in jets from young stars. Moreover there is now general agreement that magnetic fields may play a very important role not only in the formation of these jets but also their subsequent collimation. With these ideas in mind we have developed a new multidimensional magneto-hydrodynamic second order upwind code that includes the above loss terms. Fluxes at cell interfaces are calculated using a linear approximation and, if this fails, a nonlinear iterative solver. The condition ∇ ·B = 0 is maintained by including small source terms in the conservation equations. We find that the propagation dynamics and morphology of magnetised supersonic radiative jets are significantly different to their hydrodynamic counterparts even when β=8πPgas/B ≈ 1. Both steady and pulsed jets were simulated. In particular, magnetic fields for the three configurations we tested (helical, toroidal, and poloidal) enhance the jet collimation. For example, longitudinal fields restrict the lateral motion of the flow and a purely toroidal field, through hoop stresses, constricts the jet towards its axis. Such stresses, in the toroidal field case, may lead to the jet exhibiting extended nose cones, enhanced bow shock speeds, and disruption of internal working surfaces (knots) formed by velocity variations in the jet. We find that poloidal fields maintain a more stable degree of collimation and knots are not destroyed. Cooling also improves the jet collimation as it reduces the thermal support in the cocoon making it narrower than its adiabatic counterpart. Another effect of cooling is that it gives rise to Rayleigh-Taylor (RT) unstable configurations at the head of the jet causing the bow shock to periodically break up into smaller structures that sank back into the jet cocoon. This could explain some of the knotty structures seen in Herbig-Haro bows.